Apparatus, system and method for cascaded power conversion

ABSTRACT

An apparatus, method, and system are provided for power conversion to supply power to a load such as a plurality of light emitting diodes. An exemplary apparatus comprises: a first power converter stage having a first power switch and a first inductive element; a second power converter stage having a second power switch and a second inductive element; a plurality of sensors; and a controller. The second power converter stage provides an output current to the load. The controller is adapted to use a sensed input voltage to determine a switching period, and is further adapted to turn the first and second power switches into an on-state at a frequency substantially corresponding to the switching period while maintaining a switching duty cycle within a predetermined range.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/181,686, filed Jul. 29, 2008 (now U.S. Pat. No. 8,344,638).

BACKGROUND

A wide variety of off-line light-emitting diode (LED) drivers are known.For example, a capacitive drop off-line LED driver from On Semiconductor(Application Note AND8146/D) is a non-isolated driver with lowefficiency, is limited to delivering relatively low power, and at mostcan deliver a constant current to the LED with no temperaturecompensation, no dimming arrangements, and no voltage or currentprotection for the LED.

Other isolated off-line LED drivers also have wide-rangingcharacteristics, such as a line frequency transformer and currentregulator (On Semiconductor Application Note AND8137/D); a current modecontroller (On Semiconductor Application Note AND8136/D); a white LEDluminary light control system (U.S. Pat. No. 6,441,558); LED drivingcircuitry with light intensity feedback to control output lightintensity of an LED (U.S. Pat. No. 6,153,985); a non-linearlight-emitting load current control (U.S. Pat. No. 6,400,102); a flybackas an LED Driver (U.S. Pat. No. 6,304,464); a power supply for an LED(U.S. Pat. No. 6,557,512); and a voltage booster for enabling the powerfactor controller of an LED lamp upon a low AC or DC supply (U.S. Pat.No. 6,091,614).

In general, these various LED drivers are overly complicated. Somerequire control methods that are complex, some are difficult to designand implement, and others require many electronic components. A largenumber of components can increase cost and reduce reliability. Manydrivers utilize a current mode regulator with a ramp compensation in apulse width modulation (“PWM”) circuit. Such current mode regulatorsrequire relatively many functional circuits, while nonethelesscontinuing to exhibit stability problems when used in the continuouscurrent mode with a duty cycle or ratio over fifty percent. Variousattempts to solve these problems utilized a constant off-time boostconverter or hysteretic pulse train booster. While these solutionsaddressed problems of instability, these hysteretic pulse trainconverters exhibited other difficulties, such as elevatedelectromagnetic interference, inability to meet other electromagneticcompatibility requirements, and relative inefficiency. Other attempts,such as in U.S. Pat. No. 6,515,434 B1 and U.S. Pat. No. 6,747,420,provide solutions outside the original power converter stages, addingadditional feedback and other circuits, rendering the LED driver evenlarger and more complicated.

Widespread proliferation of solid state lighting systems (semiconductor,LED-based lighting sources) has created a demand for highly efficientpower converters, such as LED drivers, with high conversion ratios ofinput to output voltages. In order to reduce the component count, suchconverters may be constructed without isolation transformers by usingtwo-stage converters with the second stage running at a very low dutycycle (equivalently referred to as a duty ratio), thereby limiting themaximum operating frequency, resulting in an increase in the size of theconverter (due to the comparatively low operating frequency), andultimately defeating the purpose of removing coupling transformers.

Various proposals to solve these problems have included use of quadraticpower converters for providing a low output voltage with a wide DCconversion range, such as the quadratic power converter 10 illustratedin FIG. 1. For example, in “Switching Converter with Wide DC ConversionRange” (D. Maksimovic and S. Ćuk, May 1989 HFPC Proceedings and also inIEEE Transactions on Power Electronics, Vol. 6, No. 1, January 1991),the authors suggested using PWM converters having a single switch andfeaturing voltage conversion ratios with a quadratic dependence of theduty ratio. The cascaded buck and buck-boost topologies were designedand analytically synthesized for controlling the output voltage. Whenthese circuits are used as a current source, however, they become asinadequate as conventional one-stage converters, and exhibit even moreproblems when used with a sinusoidal input current. For example, thesecircuits require a large capacitive filter following the rectified ACsignal to continuously provide a steady DC output, thereby making powerfactor correction (“PFC”) practically impossible.

Referring to FIG. 1, the input DC voltage Vg 11 is applied to the firststage (buck-boost converter), comprising of transistor 20 (controlled bysome type of controller 21), first inductor 15, capacitor 16, and diode12. When the transistor 20 is conducting, for a linear (non-saturating)inductor 15, current is building substantially linearly in the inductor15, while diode 12 is blocked by the reverse voltage during this portionof the cycle. When the transistor 20 is off, energy stored in theinductor 15 discharges into capacitor 16, diode 12 is forward biased andconducting during part of the off-time (discontinuous mode of operation,“DCM”) or completely during the off-time (continuous mode of operation,“CCM”), and the on-off cycle is repeated. The secondary stage isillustrated as a buck converter and comprises of the transistor 20,capacitor 18, second inductor 14, and diodes 13 and 17, with the load(illustrated as resistor 19) connected across capacitor 18. When thetransistor 20 is conducting, energy from capacitor 16 is beingtransferred to the load 19 and output capacitor 18 via inductor 14, alsocharging it linearly, while diode 13 is conducting and diode 12 isblocked. When the transistor 20 is off and not conducting, diode 13 isreverse biased, and diode 17 is conducting, discharging inductor 14 intooutput capacitor 18. The operational process of the buck converter alsomay be either DCM or CCM. The transfer ratio of the converter 10 is

${- \frac{D^{2}}{1 - D}},$where D is duty cycle or ratio, with the minus sign denoting that thepolarity of the output voltage is reversed compared to the inputvoltage. Also, currents in transistor 20 and the output load are flowingin opposite directions, creating a difficult topology for sensingoperational signals and providing corresponding feedback signals (e.g.,both nodes “A” and “B” are at return potentials).

The above-referenced quadratic converter is designed to work as avoltage converter with a wide conversion ratio. Were this converter 10to be used for current control in the output load, however, variousissues may arise; for example, due to any imbalance of charges, voltagesacross capacitors 16 and 18 may not match, creating an excessive voltageacross capacitor 16, which leads either to an over-design of the powerstage or lower reliability, because this converter 10 cannot work if thevoltage across capacitor 16 is greater than Vg 11. For the same reason,this converter 10 cannot be used in the AC/DC topologies requiring powerfactor correction.

Another proposed solution in U.S. Pat. No. 6,781,351, illustrated inFIG. 2, addressed the PFC problem, providing AC/DC cascaded powerconverters having high DC conversion ratios and improved AC lineharmonics, with low input harmonic currents, a comparatively high powerfactor, and efficient operation for low voltage DC outputs. Theseconverters, however, like the quadratic converters, have floatingoperational signals, which are referenced to different nodes of thepower stage. Such floating operational signals make the provision offeedback signals to a controller extremely difficult, effectivelyrequiring custom, application-specific controllers for power management.

The input 31 is an AC voltage, rectified by a bridge 32 and furtherfiltered by a small capacitor 33. The buck-boost first stage 44 includesa blocking diode 34, which allows normal operation of the buck boost 44at any value of input voltage (at node 45), thereby creating anopportunity to provide power factor correction if the on-time of aswitch 40 is relatively constant. The second stage, a buck converter,comprises of capacitor 42, inductor 39, and diodes 38 and 41, and workssubstantially the same as the buck converter discussed with reference toFIG. 1. In order to prevent an uncontrollable rise of the voltage acrossfirst stage capacitor 36, the converter 30 uses additional components, acoupled inductor, and an additional diode (not illustrated), whichnegatively affect the economics of the converter 30. A moresophisticated control technique than PWM, also described in the patent,may address the imbalance of the capacitors' charge and prevent a highvoltage at the first capacitor stage, without adding additionalcomponents to the power stage. Though the converter 30 is improvedcompared to the converter 10 because it can operate off line using an ACinput, it still has floating operational signals, requiring excessivelycomplicated feedback connections to PWM controller 46.

Switching power converters can have high internal voltages, such as upto hundreds or thousands of volts, for example. Since power switches,capacitors, and other components may operate at high internal voltagelevels, they may be subject to voltage stress, such as an electricalforce or stress across a component that potentially may cause it tofail. Further, it is desirable for a power converter to be able tofunction properly with a range of input voltages, such as those in usein different countries. For example, standard AC power voltages canrange from a low of about 95 V in the U.S. to a high of about 264 V inEurope. As input voltage varies, switching power converters typicallyhave held output voltage at a relatively constant level by adjusting theduty ratio. This prior art strategy, however, can cause voltage stressto increase dramatically over relevant portions of the input voltagerange. Switches that are able to handle such high voltage stress may bedifficult to obtain, if available at all, or if they are available, theymay be expensive or have other undesirable characteristics such as aslow switching response, a low gain, or a high on-resistance, any ofwhich may serve to reduce conversion efficiency. These voltage stressissues may cause engineers to avoid using or developing two-stageconverters.

Accordingly, a need remains to provide a high conversion ratio converterto generate a controlled output current, with reduced voltage stress,and with a capability for control without overly complicated feedbackmechanisms. Such a converter should be optimized to run using DC as wellas AC input voltages. In addition, such a converter should providesignificant power factor correction when connected to an AC line forinput power. The converter should be able to function properly over arelatively wide input voltage range, while providing the desired outputvoltage or current, and without generating excessive internal voltagesor placing components under high or excessive voltage stress. Also, itwould be desirable to provide an LED driver controller for such aconverter, included within a system for controlling a cascaded switchingpower converter, constructed and arranged for supplying power to one ora plurality of LEDs, including LEDs for high-brightness applications,while simultaneously providing an overall reduction in the size and costof the LED driver.

SUMMARY

The representative embodiments of the present disclosure providenumerous advantages for supplying power to loads such as LEDs. Thevarious representative embodiments are capable of sustaining a pluralityof types of control over such power delivery, such as providing asubstantially constant current output. The representative embodimentsmay be provided which operate over a wide range of input voltages andwith acceptable internal voltage stress levels, further providing formore available and better component selection and longer useful life forthe selected components. The representative embodiments utilize acontrol method that provides for an accurate, stable output. Therepresentative embodiments further provide a substantiallyclose-to-unity power factor when connected to an AC line for inputpower, and further generate negligible harmonics and electromagneticinterference.

A first representative embodiment provides a system and apparatus forpower conversion, in which the apparatus is couplable to a load. Theload may be linear or nonlinear and may comprise a plurality oflight-emitting diodes. The representative apparatus comprises: a firstpower converter stage comprising a first power switch and a firstinductive element; a second power converter stage coupled to the firstpower converter stage, the second power converter stage comprising asecond power switch and a second inductive element, the second powerconverter stage couplable to provide an output current to the load; afirst sensor coupled to the second power converter stage, the firstsensor adapted to sense a first parameter of the second power converterstage; and a controller coupled to the first power switch, the secondpower switch, and the first sensor, the controller adapted to determinea switching period, the controller adapted to turn the first and secondpower switches into an on-state at a frequency substantiallycorresponding to the switching period while maintaining a switching dutycycle within a predetermined range. As used herein, such a switchingfrequency typically corresponds to or is otherwise substantially relatedto the switching period as an inverse relationship, with the switchingfrequency substantially inversely proportional to the switching period.

A representative system comprises: a plurality of light-emitting diodes;a first power converter stage having a flyback configuration andcomprising a first power switch and a transformer; a first sensorcoupled to the first power converter stage, the first sensor adapted tosense an input voltage level of the first power converter stage; asecond power converter stage having a buck configuration and coupled tothe first power converter stage, the second power converter stagecomprising a second power switch and an inductor, the second powerconverter stage coupled to the plurality of light-emitting diodes toprovide an output current to the plurality of light-emitting diodes; asecond sensor coupled to the second power converter stage, the secondsensor comprising a sense transformer and adapted to sense an outputcurrent level or a second inductive element current level; and acontroller coupled to the first power switch, the second power switch,the first sensor, and the second sensor, the controller adapted to usethe sensed input voltage to determine a switching period, the controlleradapted to turn the first and second power switches into an on-state ata frequency substantially corresponding to the switching period whilemaintaining a switching duty cycle within a predetermined range.

Another representative apparatus also couplable to a plurality oflight-emitting diodes and couplable to receive an input voltage,comprises: first, a first power converter stage comprising: a firstpower switch; a first diode; a flyback transformer having a primarycoupled to the first power switch and to the input voltage and having asecondary coupled to the first diode; a first capacitor coupled to thefirst transformer secondary and to the first diode; and a first sensoradapted to determine an input voltage level; second, a second powerconverter stage coupled to the first power converter stage, the secondpower converter stage couplable to provide an output current to theplurality of light-emitting diodes, with the second power converterstage comprising: an inductor coupled to the first diode and couplableto the plurality of light-emitting diodes; a second power switch coupledto the secondary of the first transformer; an isolation transformercoupled to a gate of the second power switch; a second diode coupled tothe second inductor; a second capacitor coupled to the inductor andcouplable to the plurality of light-emitting diodes; and a sensetransformer coupled to the second power switch; and third, a controllercoupled to the first power switch, the first sensor, the isolationtransformer and the sense transformer, the controller adapted to use thesensed input voltage to determine a switching period, and the controlleradapted to turn the first and second power switches into an on-state ata frequency substantially corresponding to the switching period whilemaintaining a switching duty cycle within a predetermined range.

In a representative embodiment, the controller is adapted to determinethe switching period as a switching interval which maintains voltagestress of the first power switch and the second power switch (and othercomponents, such as a first stage diode) below correspondingpredetermined levels. Similarly, the controller is adapted to maintainthe switching duty cycle within the predetermined range to maintainvoltage stress of the first power switch and the second power switch(and other components, such as a first stage diode) below correspondingpredetermined levels. In some representative embodiments, the switchingduty cycle is substantially constant. In addition, the representativecontroller is further adapted to turn the first and second powerswitches into the on-state and into an off-state substantiallyconcurrently.

In a representative embodiment, the first parameter is a current levelcorresponding to the output current or a second inductive elementcurrent. The first power converter stage is couplable to receive aninput voltage, and the representative apparatus may further comprise: asecond sensor coupled to the first power converter stage, the secondsensor adapted to sense an input voltage level. The first sensor andsecond sensors may be referenced to a common reference, such as a groundpotential.

The controller may be further adapted to turn the first and second powerswitches into the on-state and into an off-state with a switching perioddetermined in response to the first parameter. For example, thecontroller may be further adapted to turn the first and second powerswitches into an off-state when the first parameter has increased to afirst threshold, and to turn the first and second power switches into anon-state when the first parameter has decreased to a second threshold.

In a representative embodiment, the controller may be further adapted todetermine the first threshold and the second threshold as substantiallyrelated to a predetermined reference current level, a predeterminedcurrent variance level, a minimum input voltage level, and a sensedinput voltage level. (As used herein, “substantially related to” meansand includes a wide variety of relationships, including withoutlimitation, exact equality, substantial equality, about equal to,proportional to, inversely proportional to, is affected by, etc., andany and all combinations of such relationships (e.g., directlyproportional to a square of a first parameter and inversely proportionalto a second parameter). Similarly, as used herein, “substantiallyproportional to” means and includes a wide variety of proportionalrelationships, including without limitation, exact equality, substantialequality, about equal to, proportional to, inversely proportional to,etc., and any and all combinations of such relationships (e.g., directlyproportional to a square of a first parameter and inversely proportionalto a second parameter).) For example, the controller may be furtheradapted to determine the first threshold (I_(MAX)) as substantiallyequal to a predetermined reference current level (I_(REF)) plus anoffset term, and the second threshold (I_(MIN)) as substantially equalto a predetermined reference current level (I_(REF)) minus the offsetterm, with the offset term comprising a predetermined current variancelevel (ΔI_(L2)) multiplied by the square of a minimum RMS input voltage(V² _(LOW) _(—) _(RMS)) and divided by twice the square of an RMS valueof the sensed input voltage (2*V² _(IN) _(—) _(RMS))(I_(MAX)≈I_(REF)+(ΔI_(L2)*V² _(LOW) _(—) _(RMS)/2*V² _(IN) _(—) _(RMS)))and (I_(MIN)≈I_(REF)−(ΔI_(L2)*V² _(LOW) _(—) _(RMS)/2*V² _(IN) _(—)_(RMS))).

Representative embodiments may further comprise a memory storing alook-up table, the look-up table comprising a plurality of first andsecond thresholds corresponding to a plurality of input voltage levels.The representative controller may be further adapted to determine thefirst threshold and the second threshold by accessing the look-up tableusing the sensed input voltage.

A representative memory may also store a predetermined parameter,wherein the predetermined parameter comprises at least one of thefollowing parameters: a minimum switching period, a maximum switchingperiod, a maximum duty ratio, a minimum duty ratio, a desired outputcurrent level, a desired inductor current level, a maximum inputvoltage, a minimum input voltage, a minimum RMS input voltage, a desiredinductor ripple current, a desired output ripple current, and a maximumvoltage stress level.

In another representative embodiment, the switching period correspondsto a first threshold and to a second threshold, and wherein thecontroller is further adapted to turn the first and second powerswitches into an off-state when the first parameter has increased to thefirst threshold and into the on-state when the first parameter hasdecreased to the second threshold.

In various representative embodiments, the controller may be furtheradapted to determine the switching period in response to the sensedinput voltage level. Representative embodiments may further comprise amemory storing a look-up table, the look-up table comprising a pluralityof switching period values corresponding to a plurality of input voltagelevels. The representative controller may be further adapted todetermine the switching period by accessing the look-up table using thesensed input voltage.

In some representative embodiments, the controller may be furtheradapted to determine the switching period based on a maximum switchingperiod, a minimum input voltage, and the sensed input voltage. Forexample, the controller may be further adapted to determine theswitching period (T) as substantially proportional to a maximumswitching period multiplied by the square of a minimum RMS input voltageand divided by the square of an RMS value of the sensed input voltage(T∝T_(MAX)*V² _(LOW) _(—) _(RMS)/V² _(IN) _(—) _(RMS)), or assubstantially proportional to a minimum switching period multiplied bythe square of a maximum RMS input voltage and divided by the square ofan RMS value of the sensed input voltage (T∝T_(MIN)*V² _(HIGH) _(—)_(RMS)/V² _(IN) _(—) _(RMS)). Also for example, the controller may befurther adapted to determine the switching period (T) as substantiallyproportional to a selected switching period (T₁) multiplied by thesquare of a selected RMS input voltage (V₁ _(—) _(RMS)) and divided bythe square of an RMS value of the sensed input voltage (T∝T₁*V² ₁ _(—)_(RMS) V² _(IN) _(—) _(RMS)).

The representative controller may be further adapted to decrease theduty cycle when the output current is above a first predetermined level,and to increase the duty cycle when the output current is below a secondpredetermined level.

In a representative embodiment, the first power converter stagecomprises a flyback configuration, or a buck configuration, or adouble-buck configuration, and the second power converter stagecomprises a buck configuration, or a boost configuration, or abuck-boost configuration. In such representative embodiments, the firstinductive element may comprise a transformer and the second inductiveelement comprises an inductor, while in other representativeembodiments, the first inductive element comprises at least one firstinductor and the second inductive element comprises a second inductor.

In another representative embodiment, the controller is further adaptedto operate the first power converter stage in a discontinuous conductionmode and to operate the second power converter stage in a continuousconduction mode. In addition, the first power converter stage iscouplable to receive an input voltage having a predetermined range ofvoltages, such as substantially from 90 V RMS to 264 V RMS. The firstpower converter stage also may further comprise a rectifier couplable toreceive an AC input voltage.

In another representative embodiment, a method is disclosed forproviding power conversion for a load using a power converter comprisinga first power converter stage coupled to a second power converter stage,the first power converter stage comprising a first inductive element anda first power switch and the second power converter stage comprising asecond inductive element and a second power switch. The representativemethod comprises: sensing a first parameter comprising an input voltage;sensing a second parameter, the second parameter comprising an outputcurrent or a second inductive element current; using the sensed inputvoltage, determining a switching period; turning the first and secondpower switches into an on-state substantially concurrently and at afrequency substantially equal to the switching period; and turning thefirst and second power switches into an off-state substantiallyconcurrently while maintaining a switching duty cycle within apredetermined range.

In some representative embodiments, the switching duty cycle issubstantially constant. In various representative embodiments, theswitching period corresponds to a first threshold and to a secondthreshold, wherein the step of turning the first and second powerswitches into the off-state further comprises turning the first andsecond power switches into the off-state when the first parameter hasincreased to the first threshold; and wherein the step of turning thefirst and second power switches into the on-state further comprisesturning the first and second power switches into the on-state when thefirst parameter has decreased to the second threshold.

A representative method embodiment may further comprise using apredetermined reference current level, a predetermined current variancelevel, a minimum input voltage level, and the sensed input voltagelevel, determining the first threshold and the second threshold. Inanother representative embodiment, the method further comprises usingthe sensed input voltage level, accessing a memory to determine thefirst threshold and the second threshold; or using a sensed inputvoltage level, accessing a memory to determine the switching period. Inyet another representative embodiment, the method further comprisesusing a maximum switching period, a minimum input voltage, and thesensed input voltage, determining the switching period. Anotherrepresentative method embodiment further comprises decreasing the dutycycle when the output current is above a first predetermined level; andincreasing the duty cycle when the output current is below a secondpredetermined level.

Lastly, a representative method embodiment may further compriseoperating the first power converter stage in a discontinuous conductionmode and operating the second power converter stage in a continuousconduction mode. Such a representative method may also include receivingan input voltage having a predetermined range of voltages.

Numerous other advantages and features of the present disclosure willbecome readily apparent from the following detailed description, fromthe claims, and from the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present disclosure will be morereadily appreciated upon reference to the following description whenconsidered in conjunction with the accompanying drawings, wherein likereference numerals are used to identify identical components in thevarious views, and wherein reference numerals with alphabetic charactersare utilized to identify additional types, instantiations or variationsof a selected component embodiment in the various views, in which:

FIG. 1 is a circuit diagram illustrating a prior art quadraticconverter;

FIG. 2 is a circuit diagram illustrating a prior art cascaded converter;

FIG. 3 is a block diagram illustrating a first representative system, afirst representative regulator, and a first representative apparatus inaccordance with the teachings of the present disclosure;

FIG. 4 is a block and circuit diagram illustrating a secondrepresentative system and a second representative apparatus inaccordance with the teachings of the present disclosure;

FIG. 5 is a block and circuit diagram illustrating a thirdrepresentative system and a third representative apparatus in accordancewith the teachings of the present disclosure;

FIG. 6 is a block and circuit diagram illustrating a fourthrepresentative system and a fourth representative apparatus inaccordance with the teachings of the present disclosure;

FIG. 7 is a block and circuit diagram illustrating a fifthrepresentative system and a fifth representative apparatus in accordancewith the teachings of the present disclosure;

FIG. 8A, FIG. 8B, FIG. 8C, and FIG. 8D are graphical diagramsillustrating representative inductor current waveforms and controlsignals in accordance with the teachings of the present disclosure;

FIG. 9A and FIG. 9B are graphical diagrams illustrating representativeinductor current waveforms in accordance with the teachings of thepresent disclosure;

FIG. 10 is a block diagram illustrating a representative regulator inaccordance with the teachings of the present disclosure;

FIG. 11A and FIG. 11B are graphical diagrams respectively illustratingrepresentative inductor current (or output current) and control signalwaveforms in accordance with the teachings of the present disclosure;

FIG. 12 is a flow diagram illustrating a first method of controlling acascaded power converter in accordance with the teachings of the presentdisclosure;

FIG. 13 is a block diagram illustrating a representative controller anda representative regulator in accordance with the teachings of thepresent disclosure;

FIG. 14 is a flow diagram illustrating a second method of controlling acascaded power converter in accordance with the teachings of the presentdisclosure; and

FIG. 15 is a block and circuit diagram illustrating a thirdrepresentative controller and a fourth representative regulator inaccordance with the teachings of the present disclosure.

DETAILED DESCRIPTION

While the present disclosure is susceptible of embodiment in manydifferent forms, there are shown in the drawings and will be describedherein in detail specific representative embodiments thereof, with theunderstanding that the present description is to be considered as anexemplification of the principles of the disclosure and is not intendedto limit the disclosure to the specific embodiments illustrated. In thisrespect, before explaining at least one embodiment consistent with thepresent disclosure in detail, it is to be understood that the disclosureis not limited in its application to the details of construction and tothe arrangements of components set forth above and below, illustrated inthe drawings, or as described in the examples. Methods and apparatusesconsistent with the present disclosure are capable of other embodimentsand of being practiced and carried out in various ways. Also, it is tobe understood that the phraseology and terminology employed herein, aswell as the abstract included below, are for the purposes of descriptionand should not be regarded as limiting.

FIG. 3 is a block diagram illustrating a first representative system150, a first representative regulator 550, and a first representativeapparatus 100 in accordance with the teachings of the presentdisclosure. The system 150 comprises the apparatus 100 and a load 120,and is couplable to receive input power, such as an AC or DC inputvoltage, from input 105 which is couplable or connected to firstconverter stage 110. (AC and DC input voltages as referred to herein andwithin the scope of the present disclosure are discussed in greaterdetail following the discussion of FIG. 15). The first representativeapparatus 100 comprises the first converter stage 110, a secondconverter stage 115, a controller 500, and a plurality of sensors,illustrated as a first sensor 125, a second sensor 130, and a thirdsensor 135. (Additional sensors may also be utilized, such as anoptional fourth, fifth, and sixth sensors discussed with reference toFIG. 6, as well as a fewer number of sensors, as discussed withreference to FIG. 10.) The controller 500 may be any type of controller,processor, other integrated circuit or portion of an integrated circuit,for example, and is discussed in greater detail below with reference toFIG. 10. In addition, the controller 500 may have any number of inputsand outputs, depending on the selected embodiment.

As illustrated, the first representative apparatus 100 also typicallycomprises a gate driver 165, which may be a separate component or whichmay be considered to be part of the controller 500. The plurality ofsensors may have a common reference 155, such as being referenced to acommon node such as a ground potential. The common reference 155 issometimes illustratively shown herein as a ground potential, however itis to be understood that one or more alternative references areconsidered equivalent and within the scope of the present disclosure.Sensors 125, 130, and 135 each have one or more inputs and one or moreoutputs. Additional sensors may also be utilized. Sensors may bereferenced to common reference 155, other nodes, or they may befloating. For example, sensors may use “low-side” or “high-side”sensing. The regulator portion of the apparatus 100 (firstrepresentative regulator 550) comprises the controller 500 and thevarious sensors 125, 130, and 135. Gate driver 165 couples one or moreoutput signals from controller 500 to first and second converter stages110 and 115 and may perform one or more functions such as currentlimiting, overload protection, voltage translation, signal inversion,amplification, impedance matching, attenuation, signal conditioning,isolation, etc., and depending upon the selected configuration,additional gate drivers may also be utilized. In an representativeembodiment, gate driver 165 comprises one or more amplifiers, resistors,capacitors, transformers, and other passive and active components. Insome of the descriptions and illustrations herein, operation ofcontroller 500 may be discussed without reference to gate driver 165; itis to be understood, however, that various embodiments will utilize agate driver where appropriate. The first converter stage 110 typicallycomprises a first switch and a first inductive element such as atransformer or inductor. The second converter stage 115 typicallycomprises a second switch and a second inductive element such as atransformer or inductor. In various representative embodiments, thefirst and second converter stages 110 and 115 may share a common switch.The apparatus 100 receives the input 105, such as an AC or DC voltage.Using feedback provided by the plurality of sensors 125, 130, and 135,the controller 500 generates one or more control signals which areprovided (via the gate driver 165) to first converter stage 110 andsecond converter stage 115, such as a control signal for turning aswitch into an on and conducting state, or turning a switch into an offand substantially non-conducting state, to provide a controlled currentto the load 120, such as one or more LEDs, optionally configured as anLED array comprising one or more strings of LEDs. In a representativeembodiment, an output signal from the controller 500 (provided via thegate driver 165) comprises a series or sequence of control pulses thatturn power switches off and on in the first and second converter stages110 and 115. According to the representative embodiments of thedisclosure, this results in sourcing a substantially constant current tothe output load or, when applied to LEDs, driving a single LED, aplurality of LEDs, or an array or plurality of strings of LEDs.

It should be noted that functions shown in FIG. 3 may be combined, andin some cases, bypassed, within the scope of the present disclosure andwithout changing the operation of system 150, apparatus 100, andregulator 550. For example and without limitation, in variousrepresentative embodiments, gate driver 165 is a separate circuitelement (as illustrated in FIG. 3), while in other representativeembodiments (not separately illustrated), controller 500 directly drivesthe gates of switches in converter stages 110 and 115 and is connecteddirectly to the switches without utilizing a gate driver, or gate driver165 is incorporated into controller 500, or gate driver 165 isincorporated into first converter stage 110 and/or second converterstage 115. In the representative embodiment illustrated in FIG. 3,sensors 125, 130, and 135 are shown as separate circuit elements. Inalternative representative embodiments, such sensors may be incorporatedinto the corresponding circuitry they are configured to sense. Forexample, first sensor 125 may be implemented as part of first converterstage 110, second sensor 130 may be implemented as part of secondconverter stage 115, and third sensor 135 may be implemented as part ofload 120 or second converter stage 115. In another representativeembodiment, one or more of sensors 125, 130, 135 are implemented as partof controller 500. In addition, in FIG. 3 and in other figures shownherein, a given signal arrow may represent one or more wires or otherconductors for transmission of one or more signals.

FIG. 4 is a block and circuit diagram illustrating a secondrepresentative system 200 and a second representative apparatus 205(such as an LED driver with a cascaded converter), in accordance withthe teachings of the present disclosure. A regulator portion of theapparatus 205 is not separately illustrated, but may be considered tocomprise the controller 500 and at least some of the various sensorsdiscussed below. As illustrated, first stage 210, second stage 215A,gate driver 305, first sensor 220, and second sensor 325 are demarcatedby dotted lines and apparatus 205 is demarcated by the dot-dash line.The system 200 comprises the apparatus 205 and a load, illustrated asLED array 270, and is couplable to receive input power, such as an AC orDC input power, such as an AC input voltage V_(AC) 345 and/or a DC inputvoltage V_(IN) 355 which is couplable or connected to the first stage210. The power source V_(AC) 345 is illustratively shown as AC, but itmay alternatively be DC. As illustrated, V_(AC) 345 is converted to DCby rectifier 350 to generate V_(IN) 355. Rectifier 350 may be afull-wave rectifier, a full-wave bridge, a half-wave rectifier, anelectromechanical rectifier, or another type of rectifier. When utilizedwith a DC power source, rectifier 350 is typically not included in theapparatus 205 or system 200. The operation of the apparatus 205 will beexplained generally with reference to DC power V_(IN) 355 as an inputpower source with the understanding that rectifier 350 is used whereappropriate, depending on the nature of available power.

The representative first stage 210 is illustrated as a flyback converterand comprises a first transformer 230 (as a representative firstinductive element), a first power switch 300, a first diode 240, a firstcapacitor 245, one or more of a plurality of sensors (illustrated assensor 220 (comprising a first sense resistor 225, a second senseresistor 285, and a third capacitor 280) and a third sense resistor295), and optionally the rectifier 350. The representative second stage215A is illustrated as a buck converter and comprises a second powerswitch 250, a second diode 255, an inductor 260 (as a representativesecond inductive element), and a second capacitor 265. Switches 300 and250 are illustratively shown as N-channel MOSFETs. It should also benoted that switches 300 and 250 may be implemented as any type of powerswitch, in addition to the illustrated n-channel MOSFETs, includingwithout limitation a bipolar junction transistor, a p-channel MOSFET,various enhancement or depletion mode FETs, etc., and that a pluralityof other power switches also may be utilized in the circuitry, dependingon the selected embodiment.

A third sensor comprises a third sense resistor 295. A second sensor 325comprises transformer 360, fourth sense resistor 330, fifth senseresistor 340, and third diode 335. The first sensor 220 comprises firstsense resistor 225, second sense resistor 285, and third capacitor 280.In an alternative embodiment, an optional load current sensor 485 isprovided in series with LED array 270 and is utilized to determine theload current (“I_(LED)”) through LED array 270. Such a sensor 485 may bea sense resistor, a sensor comprising a plurality of components similarto second sensor 325, or any other type of sensor. While FIG. 4illustrates a representative embodiment with selected sensor locations,implementations, and configurations as shown, there are innumerableother sensor locations, implementations, and configurations, any and allof which are considered equivalent and within the scope of the presentdisclosure.

In FIG. 4 and elsewhere in this description, sensed, measured, orotherwise determined circuit parameters may include current through theLED array 270, LED brightness (as measured by optical sensors), voltageacross first capacitor 245, voltage across second capacitor 265, averagecurrent through the second inductive element (inductor 260 in FIG. 4),input voltage (e.g., as determined through first sensor 220), etc. Oneor more measurements may be taken and used by controller 500 to adjustany selected or predetermined operating parameters, for example, tobring the output current closer to a desired value. In a representativeembodiment, controller 500 adjusts an operating parameter comprising theamount of time switches 300 and 250 remain on during a switching cycle.

Rectifier 350 is couplable to the power source V_(AC) 345 to provideV_(IN) 355. The positive side of V_(IN) 355 is coupled to a firstterminal of the primary side of first transformer 230 and to a firstterminal of first sense resistor 225. A second terminal of first senseresistor 225 is coupled to a first terminal of second sense resistor285, to the positive side of third capacitor 280, and to input 291(“V_(IN) _(—) _(AVG)”) of controller 500. First sense resistor 225 andsecond sense resistor 285 serve as a voltage divider responsive to inputvoltage V_(IN) 355 and act in conjunction with third capacitor 280 toform a low pass filter, to average input voltage V_(IN) 355 to generatean average input voltage determination (referred to herein as “V_(IN)_(—) _(AVG)”). A second terminal of the primary side of firsttransformer 230 is coupled to the drain of first power switch 300. Thesource of first power switch 300 is coupled to a first terminal of thirdsense resistor 295 and to input 292 (“I₁”) of controller 500. A secondterminal of third sense resistor 295 is coupled to the negative side ofV_(IN) 355, a second terminal of second sense resistor 285, the negativeside of third capacitor 280, and to ground 275, which is providing acommon reference for sensor 220 and third sense resistor 295. A firstterminal of the secondary side of first transformer 230 is coupled tothe anode of first diode 240. The cathode of first diode 240 is coupledto the positive side of first capacitor 245 and to the drain of secondpower switch 250. The source of second power switch 250 is coupled tothe cathode of second diode 255, to a first terminal of inductor 260,and to a second terminal of the secondary side of third transformer 315.A second terminal of inductor 260 is coupled to the positive side ofsecond capacitor 265 and is couplable to the (positive drive side of)LED array 270.

A second terminal of the secondary side of first transformer 230 iscoupled to the negative terminal of first capacitor 245 and to a firstterminal of the primary side of second transformer 360. A secondterminal of the primary side of second transformer 360 is coupled to theanode of second diode 255, the negative side of second capacitor 265,and is couplable to the (negative drive side of) LED array 270. A firstterminal of the secondary side of second transformer 360 is coupled tothe anode of third diode 335 and a first terminal of fourth senseresistor 330. The cathode of third diode 335 is coupled to ground 275and to a first terminal of fifth sense resistor 340. A second terminalof the secondary side of second transformer 360 is coupled to a secondterminal of fourth sense resistor 330, a second terminal of fifth senseresistor 340, and input 293 (“I₂”) of controller 500.

The output of controller 500 is coupled via the driver 305 to the gatesof the power switches 300 and 250 of the respective first and secondstages 210, 215A, for control of the on and off durations of theseswitches (switching period and duty cycle). (As used herein, duty cyclewill be understood to mean and refer to the ratio of switch on-time(t_(ON)) to the switching period (“T”) (where T≈t_(ON)+t_(OFF)) or,stated another way, the proportion of the switching period during whichthe switches are in an on-state and conducting.) Gate driver 305comprises driver 290, fourth capacitor 320, fifth capacitor 310, andthird transformer 315, and is utilized, among other things, to provideisolation of the second stage 215A. The output of driver 290 is coupledto the gate of first power switch 300 and to a first terminal of fourthcapacitor 320. A second terminal of fourth capacitor 320 is coupled to afirst terminal of the primary side of third transformer 315. A secondterminal of the primary side of third transformer 315 is coupled toground. A first terminal of the secondary side of third transformer 315is coupled to a first terminal of fifth capacitor 310. A second terminalof fifth capacitor 310 is coupled to the gate of second power switch250.

In this representative embodiment, power switches 300 and 250 aresynchronized so that they turn off and on at substantially orapproximately the same time. While the switching on and off of the powerswitches 300 and 250 is generally substantially concurrent, there may bevarious and differing delays involved in the reception of controlsignals at the gates of the power switches 300 and 250, such as due tointervening drivers and use of different types of drivers for thedifferent power switches 300 and 250, for example, such as thoseillustrated in FIGS. 4, 5, and 7. Accordingly, substantially concurrentswitching of the power switches 300 and 250 should be understood to beapproximately concurrent to account for such potentially different orasymmetrical transmission delays of the control signals from controller500. When first power switch 300 is on (for the duration t_(ON)),current from the positive side of V_(IN) 355 flows through the primaryside of first transformer 230, through first power switch 300, throughthird sense resistor 295, and back to the negative side of V_(IN) 355,transferring energy from V_(IN) 355 to first transformer 230. During thesame time interval (t_(ON)), second power switch 250 is also on andconducting, so first capacitor 245 discharges through second powerswitch 250, storing energy in inductor 260 and second capacitor 265.When first power switch 300 is off (for the duration t_(OFF)), thesecondary of first transformer 230 discharges into first capacitor 245.During the same time interval (t_(OFF)), second power switch 250 is alsooff, so inductor 260 discharges into second capacitor 265 and into theload, LED array 270.

FIG. 5 is a block and circuit diagram illustrating a thirdrepresentative system 480 and a third representative apparatus 490 (suchas an LED driver with a cascaded converter), in accordance with theteachings of the present disclosure. As illustrated, the thirdrepresentative system 480 and the third representative apparatus 490differ from the respective second representative system 200 and thesecond representative apparatus 205 insofar as second power switch 250is provided on the “low side” of the second stage 215B, with the secondpower switch 250 having slightly different couplings to the driver 305as illustrated, and with the other components coupled as illustrated dueto the change in the second power switch 250 configuration in thecircuit. The first stage 210, second stage 215B, gate driver 305, firstsensor 220, and second sensor 325 are also demarcated by dotted linesand apparatus 490 is demarcated by the dot-dash line. The system 480comprises the apparatus 490 and a load, illustrated as LED array 270.

More particularly, with respect to the circuit configuration ofapparatus 490 compared to apparatus 205, a first terminal of thesecondary side of first transformer 230 is coupled to the anode of firstdiode 240. The cathode of first diode 240 is coupled to the positiveside of first capacitor 245, a first terminal of inductor 260, and thecathode of diode 255. A second terminal of inductor 260 is coupled tothe positive side of second capacitor 265 and is couplable to thepositive drive side of LED array 270.

Continuing to refer to FIG. 5, a second terminal of the secondary sideof first transformer 230 is coupled to the negative terminal of firstcapacitor 245, to a first terminal of fifth capacitor 310, and to thesource of second power switch 250. The drain of second power switch 250is coupled to a first terminal of the primary side of second transformer360. The gate of switch 250 is coupled to the second terminal of thesecondary side of third transformer 315.

In this representative embodiment of the disclosure, power switches 300and 250 are also synchronized so that they turn off and on atsubstantially the same time. When first power switch 300 is on (for theduration t_(ON)), current from the positive side of V_(IN) 355 flowsthrough the primary side of first transformer 230, through first powerswitch 300, through third sense resistor 295, and back to the negativeside of V_(IN) 355, transferring energy from V_(IN) 355 to firsttransformer 230. During the same time interval t_(ON), second powerswitch 250 is on, so first capacitor 245 discharges and stores energy ininductor 260 and second capacitor 265. When first power switch 300 isoff (for the duration t_(OFF)), the secondary of first transformer 230discharges into first capacitor 245. During the same time intervalt_(OFF), second power switch 250 is off, so inductor 260 discharges intosecond capacitor 265 and into the load, LED array 270.

FIG. 6 is a block and circuit diagram illustrating a fourthrepresentative system 400 and a fourth representative apparatus 405(such as an LED driver with a cascaded converter), in accordance withthe teachings of the present disclosure. A regulator portion of theapparatus 405 is not separately illustrated, but may be considered tocomprise the controller 500 and at least some of the various sensorsdiscussed below. As illustrated, first stage 410, second stage 415, anddriver 306 are demarcated by dotted lines and apparatus 405 isdemarcated by the dot-dash line. The system 400 comprises the apparatus405 and a load, illustrated as LED array 270, and is couplable toreceive input power, such as an AC or DC input power, as discussedabove. As illustrated, the fourth representative system 400 and thefourth representative apparatus 405 differ from the respective thirdrepresentative system 480 and the third representative apparatus 490insofar as various sensors are located and configured differently, usinga plurality of sense resistors (435, 440, 445) rather than the sensors325 and 485, a driver 306 is utilized instead of the driver 305, andwith the other components coupled as illustrated due to the changes insensor and driver configurations in the circuit.

More particularly, the first stage 410 is also a flyback converter, andin addition to the components discussed above, further comprises afourth capacitor 420 coupled to the rectifier 350 and the firsttransformer 230, and is utilized to reduce voltage fluctuations inV_(IN) 355. A gate driver 306 comprises driver 290, ninth resistor 425,and tenth resistor 430. (In one embodiment, resistors 425 and 430 areconsidered as being separate from first stage 410 and second stage 415,respectively. In an alternative embodiment (not separately illustrated),resistors 425 and 430 are considered to be part of first stage 410 andsecond stage 415, respectively.) The output of controller 500 is coupledto the input of driver 290, and the output of driver 290 is coupled to afirst terminal of ninth resistor 425 and to a first terminal of tenthresistor 430. A second terminal of ninth resistor 425 is coupled to thegate of first power switch 300, and a second terminal of tenth resistor430 is coupled to the gate of second power switch 250.

The second stage 415 is also a buck converter, and in addition to thecomponents discussed above, further comprises other sensors illustratedand embodied as a sixth sense resistor 435, a seventh sense resistor440, and an eighth sense resistor 445. Innumerable other or additionalsensor locations and configurations, any and all of which are consideredequivalent and within the scope of the present disclosure.

Continuing to refer to FIG. 6, a second terminal of the sixth senseresistor 435 is coupled to the source of second power switch 250 and toinput 296 of controller 500. The drain of second power switch 250 iscoupled to the anode of second diode 255, input 298 of controller 500,and a first terminal of the seventh sense resistor 440. A secondterminal of the seventh sense resistor 440 is coupled to the negativeside of the second capacitor 265, a first terminal of the eighth senseresistor 445, and to input 297 of controller 500. Controller inputs 297and 298 are configured to measure voltage across the seventh senseresistor 440 and may utilize a corresponding two connections, one toeach terminal of seventh resistor 440, as shown. A second terminal ofthe eighth sense resistor 445 is couplable to the negative drive side ofLED array 270 and to input 299 of controller 500. In a representativeembodiment, current (“I_(LED)”) through the load (LED array 270) isdetermined by controller 500 using the eighth sense resistor 445 (usinginputs 297, 299). Also in a representative embodiment of the presentdisclosure, controller 500 determines the current of inductor 260(“I_(L2)”) using sense resistor 440.

Sense resistors 285, 295, 435, 440, and 445 are utilized in currentsensing by developing a voltage between a first terminal and a secondterminal of each resistor and the voltages are measured by correspondingcircuitry within the controller 500. For example and without limitation,when a sense resistor is connected to a common reference such as ground,the controller 500 may determine the voltage directly; otherwise,voltage may be sensed by subtracting the voltage at a first terminal ofa sense resistor from the voltage at a second terminal of the senseresistor, or by using a “high-side” sensing technique, also for exampleand without limitation. It is to be understood that the current sensingcircuits and configuration illustrated in FIG. 6 may apply to senseresistors in other embodiments described herein, including withoutlimitation, various sensors and configurations in FIG. 3, FIG. 4, FIG.5, and FIG. 7.

In this representative embodiment of the disclosure, power switches 300and 250 are also synchronized so that they turn off and on atsubstantially the same time, and the apparatus 405 and system 400operate similarly to the apparatus 490 and system 480 discussed above.When first power switch 300 is on (for the duration t_(ON)), currentfrom the positive side of V_(IN) 355 flows through the primary side offirst transformer 230, through first power switch 300, through thirdsense resistor 295, and back to the negative side of V_(IN) 355,transferring energy from V_(IN) 355 to first transformer 230. During thesame time interval t_(ON), second power switch 250 is on, so firstcapacitor 245 discharges and stores energy in inductor 260 and secondcapacitor 265. When first power switch 300 is off (for the durationt_(OFF)), the secondary of first transformer 230 discharges into firstcapacitor 245. During the same time interval t_(OFF), second powerswitch 250 is off, so inductor 260 discharges into second capacitor 265and into the load, LED array 270.

FIG. 7 is a block and circuit diagram illustrating a fifthrepresentative system 520 and a fifth representative apparatus 505 (suchas an LED driver with a cascaded converter), in accordance with theteachings of the present disclosure. A regulator portion of theapparatus 505 is not separately illustrated, but may be considered tocomprise the controller 500 and at least some of the various sensorsdiscussed below. As illustrated, first stage 510, second stage 515, gatedriver 305, first sensor 220, and second sensor 325 are demarcated bydotted lines and apparatus 505 is demarcated by the dot-dash line. Thesystem 520 comprises the apparatus 505 and a load, illustrated as LEDarray 270, and is also couplable to receive input power, such as an ACor DC input power, also as discussed above. The apparatus 505 and system520 differ from the previously discussed apparatuses and systems withthe first converter stage 510 comprising a double buck converter, ratherthan a flyback converter, and illustrates yet an additional circuittopology which may be utilized equivalently and within the scope of thepresent disclosure.

More particularly, the apparatus 505 differs from those discussedpreviously with the first stage 510 being a double buck converter andcomprising rectifier 350; a first inductive element which, in thisconfiguration, comprises two inductive components, namely, a firstinductor 460 and a second inductor 470; a first power switch 300, aplurality of diodes (a first diode 455 and a second diode 475); aplurality of sensors (illustrated as a first sense resistor 225, asecond sense resistor 285, and a third sense resistor 295), and aplurality of capacitors (a first capacitor 465 and a third capacitor280). The second stage 515 is also a buck converter having the sameconfiguration discussed previously with reference to second stage 215Aof FIG. 4, and comprises a second power switch 250, a third diode 255, asecond inductive element (namely, a third inductor 260), and a secondcapacitor 265. The power switches 300 and 250 and various sensors alsohave the same configuration discussed previously with reference to FIG.4. Also as discussed above, while FIG. 7 illustrates a representativeembodiment with selected sensor locations, implementations, andconfigurations as shown, those having skill in the electronic arts willrecognize that there are innumerable other sensor locations,implementations, and configurations, any and all of which are consideredequivalent and within the scope of the present disclosure.

With regard to the first converter stage 510, the rectifier 350 andfirst sensor 220 are configured and function as previously discussed.The positive side of V_(IN) 355 is coupled to a first terminal of firstinductor 460, a first terminal of first sense resistor 225, and thecathode of first diode 455. A second terminal of first inductor 460 iscoupled to the positive terminal of first capacitor 465, the cathode ofsecond diode 475, and the drain of second power switch 250. The negativeterminal of first capacitor 465 is coupled to the anode of first diode455, a first terminal of second inductor 470, and a first terminal ofthe primary side of first transformer 360. A second terminal of secondinductor 470 is coupled to the drain of first power switch 300 and theanode of second diode 475. The source of first power switch 300 iscoupled to a first terminal of third sense resistor 295 and to input 292of controller 500. The remaining portions of apparatus 505 areconfigured as previously discussed with reference to FIG. 4.

In this representative embodiment of the disclosure, power switches 300and 250 are also synchronized so that they turn off and on atsubstantially the same time. When first power switch 300 is on (for theduration t_(ON)), current from the positive side of V_(IN) 355 flowsthrough the first inductor 460, first capacitor 465, second inductor470, through first power switch 300, through third sense resistor 295,and back to the negative side of V_(IN) 355, transferring energy fromV_(IN) 355 to first inductor 460, first capacitor 465, and secondinductor 470. During the same time interval t_(ON), second power switch250 is on, such that first capacitor 465 discharges through second powerswitch 250 and third inductor 260, storing energy in third inductor 260and charging second capacitor 265. When first power switch 300 andsecond power switch 250 are off (for the duration t_(OFF)), firstinductor 460 and second inductor 470 discharge into first capacitor 465and third inductor 260 discharges through third diode 255 into secondcapacitor 265. Regardless of whether the switches 250 and 300 are on oroff, power to the load (in this example, LED array 270) is provided fromeither or both second capacitor 265 and/or third inductor 260, dependingon where the controller is in the switching cycle.

It is to be understood herein that first power switch 300 and secondpower switch 250 may be implemented as any type of power switch, inaddition to the illustrated N-channel MOSFET, including, withoutlimitation, a bipolar junction transistor, an insulated-gate bipolartransistor, a P-channel MOSFET, a relay or other mechanical switch, avacuum tube, various enhancement or depletion mode FETs, etc., and thata plurality of power switches may be utilized in the circuitry. In arepresentative embodiment, these switches are turned on and offsubstantially or approximately at the same time and for the sameduration, but these switching times and durations may be different andvaried equivalently within the scope of the present disclosure. It isalso to be understood that LED array 270 is illustratively shown as astring of one or more LEDs; however, LED array 270 may comprise one ormore strings, each string comprising one or more LEDs, connected inparallel or in another arrangement. Although in a representativeembodiment the switching power converter drives one or more LEDs, theconverter is also suitable for driving other linear and nonlinear loadssuch as computer or telephone equipment, lighting systems, radiotransmitters and receivers, telephones, computer displays, motors,heaters, etc. For convenience in identifying terminals, capacitors areshown and described in illustrations and descriptions of representativeembodiments of the present disclosure as polarized; however, thecapacitors may be polarized or non-polarized.

It is also to be understood that controller 500 may have other oradditional outputs and inputs to those described and illustrated herein,and all such variations are considered equivalent and within the scopeof the present disclosure. Similarly, not all inputs and outputs forcontroller 500 may be utilized for a given embodiment of the presentdisclosure. For example, since at least one purpose of the various senseresistors is to provide input to controller 500, if some inputs are notutilized, then the corresponding sense resistors may be unused, in whichcase the unused sense resistors may optionally be eliminated. It shouldbe noted that sensors may be inserted into circuits in a plurality oflocations and configurations, using a plurality of methods, that thevarious sensors may be embodied in a wide number of ways, and that allsuch embodiments are considered equivalent and within the scope of thepresent disclosure. The illustrative embodiments shown herein describeflyback transformers without voltage snubbers; however, snubber circuitsmay be added to the flyback transformers, as desired.

FIG. 8A, FIG. 8B, FIG. 8C, and FIG. 8D are graphical diagramsillustrating representative inductor current waveforms and controlsignals in accordance with the teachings of the present disclosure,using a first stage having a flyback configuration (first transformer230 in FIG. 4, FIG. 5, and FIG. 6). In FIGS. 8A-8D, the vertical axesrepresent current “I” and the horizontal axes represent time, denoted by“t”. It should be understood that the illustrations represent an idealcase for purposes of explication and should not be regarded as limiting,and that actual measurements in practice may and likely will deviatefrom these representations. First transformer 230 has a primaryinductance L_(p). The inductance reflected to the secondary side isideally L_(s) (N_(s)/N_(p))²*L_(p), where N_(p) is the number of primarywindings and N_(s) is the number of secondary windings. The secondaryside of first transformer 230 has a first terminal connected to theanode of first diode 240 and a second terminal connected to firstcapacitor 245. In one embodiment, first transformer 230 is wound so thatcurrent flows from anode to cathode in first diode 240 when first powerswitch 300 is off. Variations of the topology, however, includingmethods of winding transformers and making corresponding alterations tothe circuitry, can be made by those skilled in the art. In analternative embodiment, first transformer 230 is wound such that currentflows in the opposite direction when first power switch 300 is on. Inthis alternative embodiment, first diode 240, as well as one or moreswitches and other components, are placed in the reverse direction,e.g., with anode and cathode switched.

In a representative embodiment of the present disclosure, it isadvantageous to have a comparatively or relatively high power factor, aspresented to input power source V_(AC) 345. One method of increasing thepower factor is to drive the first transformer 230 in the first stage indiscontinuous conduction mode (DCM). In a representative embodiment ofthe present disclosure and as illustrated in the waveforms shown inFIGS. 8A-8D, the first converter stage is operated in DCM (FIGS. 8A and8B), while the second stage is operated in continuous conduction mode(CCM) (FIG. 8C). When first power switch 300 is on, primary inductorcurrent I_(Lp) flows through the primary side of first transformer 230and into the drain of first power switch 300, as illustrated in FIG. 8A.First diode 240 connected on the secondary side is off andnon-conducting. Switches 300 and 250 share common driving signals fromcontroller 500, so second power switch 250 is also on and current I_(L2)flows from the first capacitor 245 into inductor 260, then into secondcapacitor 265 and LED array 270. Second diode 255 is off. During thisperiod of time, t_(ON), the first transformer 230 is storing energy. Atthe end of the interval t_(ON), switches 300 and 250 turn off and remainoff for time interval t_(OFF), where the switching periodT=t_(ON)+t_(OFF) is the sum of the on-time duration t_(ON) and theoff-time duration t_(OFF).

When first power switch 300 turns off, the primary current I_(Lp)becomes substantially zero, and energy is released from firsttransformer 230 through the secondary side. The secondary currentthrough first transformer 230, denoted as I_(Ls), decreases from itspeak value to substantially zero at time t_(r), where t_(r) is the resettime, as illustrated in FIG. 8B. Prior to t_(r), first diode 240conducts and second power switch 250 is off. In this period of time,t_(on)<t<t_(r), the secondary current I_(Ls) charges first capacitor245, and current from inductor 260 (current I_(L2)), which is decreasingas shown in FIG. 8C, is provided to second capacitor 265 and LED array270. At time t=t_(r), the secondary current I_(Ls) through the secondaryside of first transformer 230 substantially reaches zero and first diode240 turns off. The circuit remains in this configuration until thedriving signal turns on switches 300 and 250 again at time T, and theprocess repeats.

Since the second stage is in CCM, second inductor current I_(L2) doesnot reach zero during a switching cycle, as shown in FIG. 8C. A benefitof keeping the buck second stage in continuous conduction mode is thatthis allows a comparative reduction in the capacitance values of thesecond stage capacitors, by reducing peak-to-peak current ripple ininductor 260. The choice of the peak-to-peak inductor 260 ripple becomesa design consideration that can be selected by the designer by adjustingswitching period T and the value of inductor 260.

FIG. 9A and FIG. 9B are graphical diagrams illustrating representativeinductor current waveforms in accordance with the teachings of thepresent disclosure, and more particularly of inductor currents in therepresentative embodiment shown in FIG. 7. It should be understood thatthat the illustrations represent an ideal case for purposes ofexplication and should not be regarded as limiting, and that actualmeasurements in practice may and likely will deviate from theserepresentations. First power switch 300 and second power switch 250 areturned on at the beginning of a switching cycle by controller 500. Withfirst power switch 300 on, current flows through first inductor 460,first capacitor 465, and second inductor 470. Current I_(L460) throughfirst inductor 460 and current I_(L470) through second inductor 470 aresubstantially or effectively identical and are illustrated as such inFIG. 9A. While first power switch 300 is on for the interval t_(ON),I_(L460) and I_(L470) increase. While second power switch 250 is on(also for the interval t_(ON)), first capacitor 465 discharges intothird inductor 260, storing energy in third inductor 260 and secondcapacitor 265 and supplying power to LED array 270. While second powerswitch 250 is on, current I₁₂₆₀ through third inductor 260 increases asshown in FIG. 9B. At the end of the interval_(ON), first power switch300 and second power switch 250 are turned off by the controller 500 andremain off until the end of the current switching cycle at T. With firstpower switch 300 off for the duration t_(OFF), first inductor 460 andsecond inductor 470 discharge into first capacitor 465 (with seconddiode 475 being forward biased). With second power switch 250 off (alsofor the duration t_(OFF)), third inductor 260 discharges into secondcapacitor 265 and LED array 270. As third inductor 260 discharges,current_(L260) in third inductor 260 decreases as shown in FIG. 9B,starting at the end of the interval t_(ON) and continuing to the end ofduration t_(OFF) (and commencement of the next switching cycle at T).

As mentioned above, cascaded converters are very difficult to implementand control, largely due to voltage stress on the switch of the firstconverter stage (e.g., first switch 300), and voltage stress on aflyback diode (e.g., diode 240) when the first converter stage has aflyback configuration. In addition, such voltage stress is also highlysignificant even in a single stage converter. For example, switcheswhich could tolerate high voltages (e.g., IGBTs) are insufficientlyresponsive and too slow to be utilized in these configurations, andMOSFETs typically are not fabricated to withstand these high voltagestresses. The representative embodiments of the present disclosure bringseveral discoveries to bear to implement and control these variousmulti-stage, cascaded converter embodiments discussed above, whilesimultaneously avoiding high-voltage stresses and thereby providingactual devices which are capable of being implemented using currentlyavailable components. More particularly, in the various representativeembodiments, the switching duty ratio is maintained within predeterminedlimits or, in some embodiments, is maintained substantially constant. Inaddition, a switching period is selected which can accommodate both awide range of input voltages, such as both U.S. and European AC voltagelevels, and which can provide control over output current, whichadditionally accommodates variations in the load, such as variations inLED parameters due to temperature, aging, and other effects.

For ease of explanation, the circuit topology of FIG. 4 will be utilizedin the following discussion, with the understanding that the derivedcontrol methodology of the representative embodiments is applicable toany multi-stage converter, and is not limited to those specificallyillustrated herein. As mentioned above, controlling the first converterstage 210 to be in discontinuous conduction mode (DCM) provides asignificant benefit, among others, in that the power factor is close tounity. A large turns ratio Np/Ns, where Np is the number of primaryturns and Ns is the number of secondary turns in first transformer 230,serves to keep the first stage in DCM. Operating the first stage 210 inDCM allows the reset time to be sufficient for the flyback magnetizingcurrent to reach substantially zero after the first switch 300 turnsoff. Increasing the turns ratio Np/Ns, however, has the undesirableeffect of increasing the voltage stress of the first stage switch(switch 300).

It is also desirable for the multi-stage converter to be capable ofoperating across a range of input voltages, for example, to be useablein the U.S. and other countries. In one example of a specification for auniversal input power factor correction circuit, the source input V_(AC)345 may range from 90 V RMS to 264 V RMS. One of the novel discoveriesutilized in the representative embodiments concerns tight control overany changes in the duty ratio “D,” with other forms of control utilizedto maintain a steady output current across a range of input voltages.For this discussion, “D_(max)” is the maximum allowable duty ratio and“D_(min)” is the minimum duty ratio which may be specified or otherwisepredetermined. For high values of D_(max), a high turns ratio Np/Ns isused to keep the first converter stage in DCM, which in turn causes highvoltage stress across first switch 300, as mentioned above. For example,with the input voltage range mentioned above, if Dmax is set at 0.85,the corresponding turns ratio of Np/Ns that will keep the first stage inDCM is seven or greater, which creates primary voltage stress acrossfirst switch 300 of around 2.5 kV, which is generally unacceptable forany type of MOSFET power switch. Conversely, making Dmax comparativelysmall (which also keeps Dmin small) increases voltage stress on thesecond stage (215A, 215B) switches and diodes (e.g., second switch 250and second diode 255). Since high-voltage switches are expensive andhave other undesirable characteristics such as low efficiency and lackof responsiveness, it is desirable to keep the voltage stress on all theswitches as low as reasonably possible for a selected converterconfiguration.

The representative embodiments, therefore, utilize a very differentcontrol methodology, namely, changing the switching frequency (switchingperiod “T”) in response to changes in the output (load) current, such asthose due to temperature fluctuations, aging, and so on, and in responseto changes in input voltage (or to accommodate a wide range of inputvoltages). With these changes in switching period, the duty ratio (dutycycle) is maintained within a predetermined range or, in someembodiments, is maintained substantially constant at a predeterminedlevel. In this way, a duty ratio may be predetermined which, whenimplemented, does not cause undue voltage stress across the first andsecond stage components, while simultaneously allowing a substantiallyhigh power factor (e.g., close to unity), and further providing foraccommodation of a wide range of input voltages. For example, a dutyratio may be selected of about 0.5 (or within a range of 0.45 to 0.55,also for example), resulting in a first stage switch stress of about800-900 V, which is well within switch specifications (which may befurther reduced using a suitable snubber circuit), and which may beutilized with an input voltage range of 90 V RMS to 264 V RMS, forexample.

FIG. 10 is a block and circuit diagram illustrating a representativeregulator 600 in accordance with the teachings of the presentdisclosure. As illustrated, the regulator 600 comprises a controller500A and a plurality of sensors, illustrated as a first sensor 645 forsensing a first circuit parameter and a second sensor 635 for sensing asecond circuit parameter. For example, first sensor 645 may be an inputvoltage sensor which may be implemented in a plurality of ways todetermine the first circuit parameter, such as a voltage level,including as first sensor 220, which provides an average input voltagelevel. Also for example, the first parameter may be V_(IN) 355, V_(IN)_(—) _(AVG) as discussed above, the root mean square of V_(IN) 355 (orV_(AC) 345) (referred to herein as “V_(IN) _(—) _(RMS)”), or otherderivations of the input voltage. Similarly, the second sensor 635 maybe an output current sensor, and may be implemented as a sensor 485(e.g., a sense resistor) couplable to the load (e.g., LED array 270) forsensing a second parameter, in this case the output current through theload, or may sense another circuit parameter which may be proportionalor otherwise related to the output current, such as by using the secondsensor 325. Not separately illustrated is any clocking circuitry whichmay be utilized, e.g., clock, oscillator, etc.

The controller 500A may be implemented in a wide variety of ways, asdiscussed in greater detail below, following the discussion of FIG. 15.As illustrated, the controller 500A comprises control logic 650 and oneor more comparators 655 and/or 665 and, optionally, may include othercomponents as discussed below. The control logic 650 may comprise anytype of analog or digital circuitry for use in performing any of thevarious determinations or calculations described herein, such as one ormore adders, multipliers, finite state machines, etc., or otherwise usedin performing the various methodologies of the present disclosure, andfurther may be embodied or implemented generally as described in greaterdetail below for a controller 500, such as a processor, FPGA, ASIC,finite state machine, etc. Comparators 655 and/or 665 are utilized tocompare a value of a sensed parameter with a predetermined, selected, orcalculated value, for use in determining the switching period or dutycycle, as discussed in greater detail below. Optionally in selectedembodiments, when the controller 500A is implemented in digital form,analog-to-digital converters (“A/D”) 615 may also be utilized to convertanalog signals from the first and second sensors 645, 635 to a digitalcounterpart suitable for use by the controller 500A, with at least onedigital-to-analog converter (“D/A”) 680 then also utilized in thisselected embodiment to convert digital signals from the controller 500Ato an analog counterpart suitable for use for driving the gates of thepower switches 300, 250 and any intervening driver circuitry (290, 305,306). As mentioned above, the controller 500A may have any number ofinputs and outputs and, as illustrated, also receives input from amemory 630, which may be any type or kind of memory circuit, also asdiscussed in greater detail below. Also optionally in selectedembodiments, an input-output (“I/O”) interface 675 may also be includedwithin regulator 600, such as for programming or configuring thecontroller 500A and/or for storing information in the memory 630, suchas threshold values or a look-up table, for example and withoutlimitation. An output of the controller 500A is coupled to one or moredrivers 290 or other form of gate drivers 305 or 306, for effectiveswitching of the first and second power switches 300, 250.

Also for example, in a representative embodiment, the first parameterfrom first sensor 645 is an input voltage so that an output from thefirst sensor 645, e.g., V_(INPUT), is an indicator or otherwiserepresents the input voltage V_(IN) 355. Similarly, in a representativeembodiment, the second parameter is an output current so that the outputfrom the second sensor 635, e.g., I_(LOAD), is an indicator or otherwiserepresents the load current in this case. As indicated above, othercircuit parameters may also be determined which may also be significantand which may be utilized for the desired or selected regulation. Forexample, a sensed circuit parameter is considered to represent the loadcurrent if it is related to the output current directly enough thatregulating that selected parameter will satisfactorily regulate theoutput current for the selected application or tolerance. For example, asensed current through the second inductive element (current_(L2)) maybe utilized as an indicator of the load current. Continuing with theexample, the output from the second sensor 635 (I_(LOAD)) may be arational multiple/fraction of the output load current or it may be aparameter that is approximately or substantially proportional orotherwise related to the output load current, and similarly the outputfrom the first sensor 645 (V_(INPUT)) may be a rationalmultiple/fraction of the input voltage or it may be a parameter that isapproximately proportional or otherwise related to the input voltage.For example, sensed value I_(LOAD) output from second sensor 635 may beor represent I_(LED) from FIG. 4, 5, or 7; the current through orvoltages across resistors (or sensors) 435, 440, 445, or 295 from FIG.6; second inductive element current I_(L2); or another parameter thatrepresents load current. It is to be noted that the sensed parameter maybe a voltage, even though the parameter represents a current, forexample, and vice-versa. It should also be noted that output currentmay, in some cases, be measured and, in other cases, be determined inother ways. For example, in FIGS. 4, 5, and 7, a sensor isillustratively positioned to measure current in the second power switch250. This current is approximately equal to the current of secondinductive element 260 while power switch 250 is closed and may be usedto compare to the upper threshold I_(MAX) (explained below). While powerswitch 250 is open, other methods may be used to determine or estimateinductor current, for example, the method described in Dongsheng Zhou etal., U.S. Pat. No. 7,880,400, filed Sep. 21, 2007, entitled “DigitalDriver Apparatus, Method And System For Solid State Lighting” (the “Zhouapplication”), incorporated herein by reference with the same full forceand effect as if set forth in its entirety herein. In addition, thereare many methods for converting a current measurement into a voltagemeasurement (and vice-versa), one representative method utilizing asense resistor, optionally followed by an amplifier, for example.

There are several control methodologies within the scope of the presentdisclosure, and all serve to determine and/or vary a switching period(equivalently, switching frequency) of the multi-stage converter. In afirst method, average load or output current is also regulated usinghysteretic control, using minimum and maximum values for load/outputcurrent. In this first method, the controller 500A turns the switchesoff when the measured parameter reaches (e.g., has increased to) themaximum threshold, and turns the switches on when the measured parameterreaches (e.g., has decreased to) the minimum threshold. The maximum andminimum thresholds may be predetermined (and, for example, stored inmemory 630), or may be determined by the controller 500A, such as basedon another parameter, such as input voltage. Accordingly, setting ordetermining the maximum and minimum thresholds correspondingly affectsthe switching frequency, such that if the maximum and minimum thresholds(bounds) are comparatively far apart, the switching frequency isdecreased, and if the maximum and minimum thresholds (bounds) arecomparatively close, the switching frequency is increased. In this firstmethod, the duty cycle (duty ratio) is maintained within a predeterminedrange (or substantially constant).

In a second method, the controller 500A utilizes a sensed input voltageto determine the switching period, and then utilizes feedback from theoutput or load current to provide more precise or fine-grainedregulation of the output current, while also maintaining the duty cycle(duty ratio) within a predetermined range (or substantially constant).

This first method is illustrated graphically in FIG. 11A and FIG. 11B,which are graphical diagrams respectively illustrating representativeinductor current and/or output current (FIG. 11A) and control signal(FIG. 11B) waveforms in accordance with the teachings of the presentdisclosure. It should be noted that the current I_(L2) through secondinductive element (for example, inductor 260, or more generally, thepower-carrying inductive element in the second stage of the powerconverter, and which may be denoted as “L₂”) is, on average,substantially proportional or otherwise related to the current I_(LED)through the load (LEDs 270). In accordance with representativeembodiments, by controlling I_(L2), the controller 500A also controlsthe output current, in this case I_(LED). Therefore, in a representativeembodiment of the present disclosure, inductor current of the secondstage I_(L2) is sensed by the second sensor 635, with resulting valuesutilized by the controller 500A to control the output current. Asillustrated in FIGS. 11A and 11B, when the power switches 300, 250 areturned on by the controller 500A (interval t_(ON) 671), the current(second inductive element current or load current) will increase. Whenthe current (second inductive element current or load current) hasincreased substantially to a first threshold (illustrated as “I_(MAX)”673), the controller 500A turns the power switches 300, 250 into an offstate (non-conducting) (interval t_(OFF) 672), at which point thecurrent (second inductive element current or load current) willdecrease. When the current (second inductive element current or loadcurrent) has decreased substantially to a second threshold (illustratedas “I_(MIN)” 674), the controller 500A turns the power switches 300, 250into an on state (conducting 676), at which point the current (secondinductive element current or load current) will increase again, and theprocess repeats.

From the above description, it is apparent that the switching period T,the average value of inductor current I_(L2), the average value ofoutput current I_(LED), and the output ripple depend on values chosenfor first and second thresholds I_(MAX) and I_(MIN). In a representativeembodiment, these first and second thresholds are determined by thecontroller 500A in response to a sensed input voltage, using V_(INPUT)from first sensor 645, for example. This may be a computation using, forexample, control logic within the controller 500A. In anotherrepresentative embodiment, using a sensed parameter such as the inputvoltage, the controller 500A accesses a look-up table (LUT) stored inthe memory 630, and reads corresponding stored (and predetermined)values to determine first and second thresholds I_(MAX) and I_(MIN).

The values for the first and second thresholds I_(MAX) and I_(MIN) maybe determined by the controller (e.g., computed, read from memory, orotherwise determined) using a wide variety of input information, whichmay be sensed or stored in memory, including without limitation:

-   -   1. V_(LOW) _(—) _(RMS)—The minimum RMS (root mean square)        voltage expected for V_(IN) 355 (or V_(AC) 345);    -   2. V_(HIGH) _(—) _(RMS)—The maximum RMS (root mean square)        voltage expected for V_(IN) 355 (or V_(AC) 345);    -   3. ΔI_(L2) (or ΔI_(LED))—The selected or desired amount of        ripple current for second inductive element L₂ or the output        current I_(LED), which determines the amount of allowable        variance of the current from an average value (i.e., how far        above and below the average value I_(L2) is allowed to vary);    -   4. T_(MAX)—The maximum desired switching period;    -   5. T_(MIN)—The minimum desired switching period; and/or    -   6. I_(REF)—The desired average load current (such as the current        through LED array 270).

In a first representative embodiment of controller 500A and regulator600, I_(MAX) and I_(MIN) are set to (Equation 1):I _(MAX) ≈I _(REF)+(ΔI _(L2) *V ² _(LOW) _(—) _(RMS)/2*V ² _(IN) _(—)_(RMS)),and (Equation 2):I _(MIN) ≈I _(REF)−(ΔI _(L2) *V ² _(LOW) _(—) _(RMS)/2*V ² _(IN) _(—)_(RMS)),in which V_(IN) _(—) _(RMS) is the RMS voltage of V_(IN) 355 (or V_(AC)345). Similar equations may be derived using values for V_(HIGH) _(—)_(RMS) or other parameters. It should be noted that the first and secondthresholds I_(MAX) and I_(MIN) are determined based on a sensed valuefor an input voltage, an allowable amount of variance in output (and/orsecond inductive element) current, and an expected or predeterminedminimum or maximum input voltage level (which may be an RMS or othervalue). Since it may be convenient to scale one or more of I_(REF),I_(MIN), or I_(MAX), we can replace the substantially equal to signs(“≈”) in Equations 1 and 2 with a proportional relationship (“∝”), orwith a strict equality (“=”), or more generally as “substantiallyrelated to.” The computations may be simplified by first computing anoffset term (the portion of Equations 1 and 2 in parenthesis), thenadding the offset term to I_(REF) to obtain I_(MAX) and subtracting theoffset term from I_(REF) to obtain I_(MIN). These parameters may beutilized in a LUT in memory 630 to obtain corresponding values for thefirst and second thresholds I_(MAX) and I_(MIN).

One benefit of controlling the power converter as shown in Equations 1and 2 is that the switching period changes in response to I_(MAX) andI_(MIN) to a value that holds the output current within a predeterminedvariance while also holding the duty ratio within a predetermined rangeor variance. By holding the duty ratio within a predetermined range (orsubstantially constant), the voltage stress is maintained below apredetermined level. Using the values determined for first and secondthresholds I_(MAX) and I_(MIN) in Equations 1 and 2, the power converter(205, 490, 405, 505) is adapted to operate with a switching period andduty cycle that maintains voltage stress below a predetermined level.

Any given sensed parameter may be transformed into another comparablevalue. For example, the representative method of measuring V_(IN) _(—)_(AVG) using sensor 220 measures the average of V_(IN), yet Equations 1and 2 use V_(IN) _(—) _(RMS). There are several alternatives forobtaining a value for V_(IN) _(—) _(RMS) for use in Equations 1 and 2.In a representative embodiment, V_(IN) is multiplied by a constant “α”so that an estimate for V_(IN) _(—) _(RMS) is V_(IN) _(—) _(RMS)≈αV_(IN)_(—) _(AVG), where α is chosen to compensate for the difference betweenaverage and RMS voltage and for the voltage drop across the resistivedivider comprising first sense resistor 225 and second sense resistor285. In another representative embodiment, V_(IN) is used instead ofV_(IN) _(—) _(RMS) in Equation 1. In another representative embodiment,a sensor designed to measure RMS voltage directly is added to the powerconverter, using circuitry as known or becomes known in the electronicarts.

FIG. 12 is a flow diagram illustrating the representative first methodof controlling a cascaded power converter in accordance with theteachings of the present disclosure. As indicated above, this firstmethod of controlling a two-stage, cascaded power converter (205, 490,405, 505) involves a hysteretic process of regulating the output DCcurrent by increasing or decreasing the power switches (300, 250)on-time and off-time durations in response to at least one sensedparameter, such as a first parameter (e.g., an input voltage, such asfrom first sensor 645), and/or a second parameter (e.g., an output (orsecond inductive element) current, such as from second sensor 635).Beginning with start step 700, the converter is started, with both powerswitches' (e.g., 300, 250) turned into an “on” state. In step 705, afirst parameter is determined, such as a voltage value representing RMSinput voltage, or an approximation from the average input voltage, oranother similar metric as discussed above. In step 710, a secondparameter is determined, such as an output current level or secondinductive element current level. Typically, the first and secondparameters will be measured continuously or periodically (e.g.,sampled), for as long as the converter is in operation, for ongoing usein a plurality of comparison steps. In step 715, a first, upperthreshold is determined (e.g., I_(MAX)), and in step 720, a second,lower threshold is determined (e.g., I_(MIN)), such as through accessinga memory 630 or a calculation by the controller 500A. The value of thesecond parameter is compared with the first, upper threshold, step 725.When the value of the second parameter is greater than or substantiallyequal to the first, upper threshold, step 730, the method proceeds tostep 735, and the power switches (300, 250) are turned into an off,non-conducting state (by the controller 500A and any interveningdrivers). When the value of the second parameter is not greater than orsubstantially equal to the first, upper threshold in step 730, themethod then compares the value of the second parameter with the second,lower threshold, step 740. When the value of the second parameter isless than or substantially equal to the second, lower threshold, step745, the method proceeds to step 750, and the power switches (300, 250)are turned into an on, conducting state (by the controller 500A and anyintervening drivers). When the value of the second parameter is not lessthan or substantially equal to the second, lower threshold in step 745,indicating that the second parameter is in between the first and secondthresholds, and following steps 735 and 750, the method returns to step705, and the process repeats (until an operator switches off the inputpower to the converter).

It should be noted that, using this hysteretic process of currentregulation, the switching period responds to the values of the upper andlower thresholds. For example, if the thresholds move closer together,the switching period will tend to become shorter. Since the upper andlower thresholds are set using the input voltage, it is apparent thatthe switching period is a function of, possibly along with otherfactors, the input voltage. It should be noted that the variousoperational steps of FIG. 12 may occur in a wide variety of orders, inaddition to or in lieu of the order illustrated in FIG. 12. For example,the various comparison steps will typically occur concurrently, such asusing the representative regulator 605 illustrated in FIG. 13.

FIG. 13 is a block diagram illustrating a representative controller 500Band a representative regulator 605 in accordance with the teachings ofthe present disclosure, which may be utilized to implement the first,hysteretic control methodology discussed above. The regulator 605comprises controller 500B, a first (e.g., input voltage) sensor 645, anda second (e.g., output or inductor current) sensor 635. Controller 500Bcomprises control logic block 650, first comparator 655, secondcomparator 665, and flip-flop (or latch) 670, and optionally may alsoinclude memory 630 or be coupled to a separate memory 630. First sensor645 determines a first parameter, such as any of the various inputvoltage levels discussed above, e.g., RMS input voltage, a scaledversion of V_(IN) 355, an average version of V_(IN) 355, etc. Secondsensor 635 determines a second parameter, such as output current orinductor 260 current, as described above. The control logic block 650receives a value of the first parameter, such as an input voltage level,from first sensor 645 and determines the first and second thresholdsI_(MAX) and I_(MIN), such as through a calculation (e.g., using adders,multipliers, etc., within the control logic block 650) and/or usinginformation stored in memory 630, such as using an LUT as discussedabove. The control logic block 650 provides the corresponding first andsecond thresholds I_(MAX) and I_(MIN) to the first and secondcomparators 655, 665, respectively, which in turn compare the sensedcurrent level (provided by second sensor 635) to the first and secondthresholds I_(MAX) and I_(MIN), and provide a corresponding signal tothe flip-flop or latch 670. When the sensed current level is equal to orgreater than the first threshold I_(MAX), the output of comparator 655goes high and resets flip-flop 670, which outputs a control signal, inthis case a low (logic zero) signal, to the gate driver 290 to turn thepower switches 300, 250 into an off state. When the sensed current levelis equal to or less than the second threshold I_(MIN), the output ofcomparator 665 goes high and sets flip-flop 670, which outputs a controlsignal, in this case a high (logic one) signal, to the gate driver 290to turn the power switches 300, 250 into an on state.

Referring again to FIG. 10, the controller 500A may also implement asecond method, as mentioned above, to control the switching period(frequency) and duty cycle of the cascaded, multi-stage converter (205,490, 405, 505). As illustrated in FIG. 8D, controller 500A generates aseries of control signals (pulses) with a duty ratio of D=t_(ON)/T and aswitching period (cycle time) of T. The control signals control thepower switches 300, 250 in the first and second power converter stages,respectively, as discussed above.

In this representative embodiment, controller 500A determines aswitching period in response to a sensed input voltage (e.g., V_(INPUT))(and optionally in response to other parameters). A switching period Tthat reduces voltage stress in the power converter is proportional orsubstantially equal to (Equation 3):T≈T _(MAX) *V ² _(LOW) _(—) _(RMS) /V ² _(IN) _(—) _(RMS)(or T∝T _(MAX)*V ² _(LOW) _(—) _(RMS) /V ² _(IN) _(—) _(RMS));or, equivalently (Equation 4):T≈T _(MIN) *V ² _(HIGH) _(—) _(RMS) /V ² _(IN) _(—) _(RMS)(or T∝T _(MIN)*V ² _(HIGH) _(—) _(RMS) /V ² _(IN) _(—) _(RMS)).

The factors T_(MAX)*V² _(LOW) _(—) _(RMS) and T_(MIN)*V² _(HIGH) _(—)_(RMS) are theoretically equal, and these parameters may be used as amatter of convenience. Any line voltage (“V₁”) may be utilized withacceptable switching period (T₁) which provides the same duty ratio asdesired, such that T₁*V² ₁≈T_(MAX)*V² _(LOW)≈T_(MIN)*V² _(HIGH) _(—)_(RMS). For such a more general case, T≈T₁*V² ₁/V² _(IN) _(—) _(RMS) (orT∝T₁*V² ₁/V² _(IN) _(—) _(RMS)). In addition, various parameters orfactors may be combined and stored in a memory 630 as a singleparameter, such as storing the factor substantially equal to T_(MIN)*V²_(HIGH) _(—) _(RMS), without having T_(MIN) or V_(HIGH) stored.

Using Equation 3 or Equation 4, the switching period is determined to bea value that maintains voltage stress below a predetermined level for agiven input voltage level, allowing use of the various converters with awide range of input voltage levels. In representative embodiments,controller 500A may determine the switching period T using analog ordigital circuitry. In another representative embodiment, controller 500Adetermines the switching period T using a look-up table (LUT)comprising, for example, values for T with respect to V_(IN) _(—) _(RMS)and stored values for V_(HIGH) _(—) _(RMS) or V_(LOW) _(—) _(RMS), wherethe LUT is advantageously stored in a memory 630. Other methods fordetermining switching period T (or a corresponding switching frequency)may be used, all of which are considered essentially equivalent andwithin the scope of the present disclosure.

In a representative embodiment, the duty cycle D may be determined usingmethods known or which become known in the electronic arts to achievedesired output current, to meet reasonable electronic componentspecifications, and to meet other design goals. Modeling may also beperformed to determine a range of duty cycles which will maintainvoltage stresses within predetermined limits for corresponding componentspecifications (e.g., turns ratios). In another representativeembodiment, D is predetermined or otherwise set to an initial value,then modified slowly in response to sensed output current (or, in thecase of LEDs, optionally in response to light output) in order tocorrect for output drift in response to factors such as LED aging,temperature, etc. In a representative embodiment, if LED current exceedsa desired level, the controller 500A is adapted to slowly decrease (ordecrement) the duty cycle D until the sensed current drops to a desiredlevel. If the sensed current is too low, the controller 500A is adaptedto slowly increase the duty ratio D until the sensed current hasincreased to the selected or predetermined current level. This may beaccomplished by comparing the sensed current with a predeterminedcurrent level to generate a corresponding error signal, which is thenutilized to increase (or increment) or decrease (or decrement) the dutycycle D, as needed, for the selected switching period or frequency. Aswith determination of T (above), determination of D and generation ofcontrol pulses may, within the scope of the present disclosure, useanalog, digital, or any of a number of other methods that accomplish asimilar result. In a representative embodiment, the selected orpredetermined current level has a single value, while in anotherrepresentative embodiment, the selected or predetermined current levelcomprises multiple values or thresholds, for example, an upper and lowerbound, such as to also provide hysteretic control.

FIG. 14 is a flow diagram illustrating a second method of controlling acascaded power converter in accordance with the teachings of the presentdisclosure. As mentioned above, this second method of controlling amulti-stage power converter (205, 490, 405, 505) involves determining aswitching period T, which is variable and adjusted or otherwisedetermined in response to a sensed first parameter, such as an inputvoltage (such as an input voltage determined by first sensor 645).Beginning with start step 800, power switches 300, 250 are turned on,and a first parameter such as the input voltage is determined, step 805,as discussed above, and as may be needed, converted or approximated toan RMS value, also as discussed above. In step 810, a second parametermay also be determined, as an option, such as an output (or inductor260) current level. Typically, the first parameter (such as an inputvoltage) and second parameter (such as an output or inductor current)will be measured continuously or periodically (e.g., sampled), for aslong as the converter is in operation, for ongoing use in adjusting theswitching period or duty cycle, as may be desirable. In step 815, aswitching period T is determined, such as by using Equations 3 or 4, orthrough use of a look-up table or other memory access, for example andwithout limitation. In step 820, a duty cycle D is determined, which asmentioned above, may be selected from a predetermined range of values orlimits, may be selected as a single initial value, may be calculated ordetermined through use of a look-up table or other memory access, forexample and without limitation. In some circumstances, the duty cycle Dmay also be determined based upon a sensed parameter such as the inputvoltage.

The controller 500A generates a series of control signals (pulses) withduty ratio (duty cycle) D and switching period T. More particularly, theon-time t_(ON) of each control signal (or pulse) is a function of theduty ratio and of the switching period, as described above and asillustrated in the various figures. In step 825, the second parameter(for example, LED current, inductor current, light output, etc.) iscompared to one or more selected or predetermined levels or thresholds.When the second parameter is greater than (or substantially equal to) afirst selected or predetermined level in step 830, the duty cycle isdecreased (slowly or slightly decremented), step 835, for use in thenext switching cycle. When the second parameter is less than (orsubstantially equal to) a second selected or predetermined level in step840, the duty cycle is increased (slowly or slightly incremented), step845, also for use in the next switching cycle. As mentioned above, thefirst and second selected or predetermined levels may be maximum andminimum threshold values. If the first and second selected orpredetermined levels are the same single value, then during steady stateoperation, the duty cycle is likely to converge to the single selectedor predetermined level, or is likely to converge about the selected orpredetermined level, e.g., will then be slightly incremented for aswitching cycle, followed by slightly decremented for a next switchingcycle, followed by slightly incremented for the next switching cycle,etc. In both cases, the duty cycle will generally be stably maintainedat a constant value or within a predetermined range of values, and inboth cases, providing for reduced voltage stress in the power switches300, 250 and other components within the converter (205, 490, 405, 505).Following these determinations, the revised (or the same) duty cyclewill be utilized in the next switching cycle, step 850, the methodreturns to step 805, and the process repeats (until an operator switchesoff the input power to the converter).

As another representative embodiment illustrated in FIG. 14, instead ofvarying the duty cycle in steps 835 or 845, the duty cycle may bemaintained substantially constant, and the switching period T may becorrespondingly decremented or incremented, with any revised or newswitching period used for the next switching cycle in step 850.

As another representative embodiment not separately illustrated in FIG.14, instead of utilizing the revised switching period T or duty cycle inthe next switching cycle, a determination may be made concerning thecurrent state of the converter (whether it is still during a t_(ON)interval), and if so, the on-time duration may be varied as may beneeded during the current switching cycle, with the off-time durationvaried as needed for the commencement of the next switching cycle.

It should be noted that, as mentioned above, using this second processof current regulation, particularly with first and second thresholds,the switching period or the duty cycle responds to the values of theupper and lower thresholds. For example, when the duty cycle ismaintained substantially constant, if the thresholds move closertogether, the switching period will tend to become shorter. Since theupper and lower thresholds are set using the input voltage, it isapparent that the switching period is a function of, possibly along withother factors, the input voltage. Also, for example, when the switchingperiod is maintained substantially constant, if the thresholds movecloser together, the duty cycle will tend to become smaller. It shouldbe noted that the various operational steps of FIG. 14 may occur in awide variety of orders, in addition to or in lieu of the orderillustrated in FIG. 14.

FIG. 15 is a block and circuit diagram illustrating a thirdrepresentative controller 500C and a fourth representative regulator 900in accordance with the teachings of the present disclosure. Theregulator 900 comprises controller 500C, first sensor 645 (e.g., aninput voltage sensor), second sensor 635 (e.g., an output or inductorcurrent sensor), and also may be considered to include gate drivercircuitry (290, 305, 306). Controller 500C comprises a low pass filter915, memory 630, capacitor 930, resistor 940, error amplifier 925,comparator 655, ramp generator 950, flip-flop (or latch) 670, controllogic (block) 650, and pulse generator 970. Not separately illustratedis any clocking circuitry, as mentioned above.

The first sensor 645 and the second sensor 635 function as describedabove. The output of second sensor 635 is coupled to the input of lowpass filter 915, such as to filter any output current ripple. A selectedaverage output or inductor 260 current value is stored in memory 630,and may be predetermined and preloaded into memory 630 or it may bedynamically generated and stored in memory 630. The output of memory 630is coupled to the non-inverting input of error amplifier 925. The outputof the low pass filter 915 is coupled to the inverting input of erroramplifier 925 and a first terminal of capacitor 930. The output of erroramplifier 925 is coupled to a first terminal of resistor 940 and to theinverting input of comparator 655. A second terminal of resistor 940 iscoupled to a second terminal of capacitor 930. An output of a rampgenerator 950 is coupled to the non-inverting input of comparator 655.The output of comparator 655 is coupled to the reset input of flip-flop670. The output of first sensor 645 is coupled to the control logic 650.An output of the control logic 650 is coupled to the input of pulsegenerator 970. The output of pulse generator 970 is coupled to the setinput of flip-flop 670 and to the ramp generator 950. The output offlip-flop 670 is coupled to gate driver circuitry (290, 305, 306) fordriving the gates of power switches 300, 250 in the first and secondconverter stages.

The control logic 650 determines a value for the switching period Tbased on sensed input voltage from first sensor 645 as previouslydiscussed. The pulse generator 970 generates a series of pulses at thecorresponding switching frequency (i.e., a frequency substantially equalto 1/T) or, stated another way, generates a pulse corresponding to thestart of a switching period, such that at the beginning of each pulse, anew switching cycle begins. (The point designated as the start of a newswitching cycle is chosen merely for convenience in describing operationof the regulator 900. Another point may be chosen within the scope ofthe present disclosure.) At the beginning of the switching cycle, acorresponding pulse from pulse generator 970 resets ramp generator 950(returning its output to substantially zero) and sets flip-flop 670,driving the flip-flop 670 output high, which in turn (via gate drivercircuitry (290, 305, 306) turns the power switches 300, 250 into an onand conducting state.

The second sensor 635 senses a first parameter, such as an output orinductor 260 current, as described above, and provides a correspondingcurrent level to low pass filter 915, which averages the current level(or signal) to generate an average output (or inductor) current level.Error amplifier 925 compares the average current level from low passfilter 915 to a selected or predetermined (output or inductor) currentlevel from memory 630, and provides a corresponding output error signal(i.e., the error amplifier 925 determines a difference (as an error)between the average current level from low pass filter 915 to a selectedor predetermined (output or inductor) current level from memory 630,with the difference indicated by the corresponding DC level of theoutput error signal provided to the comparator 655). Resistor 940 andcapacitor 930 (also referred to as compensation resistor 940 andcompensation capacitor 930) are utilized to maintain a stable speed orrate of the error amplifier 925. Ramp generator 950 begins a positiveramp at the start of a switching cycle and is reset by pulse generator970 to return to a minimum value (e.g., substantially zero) for thecommencement of a next switching cycle. In a representative embodimentof the current disclosure, the ramp speed of error amplifier 925 isorders of magnitude slower than that of ramp generator 950. In anotherembodiment, the ramp speed of error amplifier 925 is set to be slowenough to preserve circuit stability and fast enough to track changes inoutput current, such as changes caused by temperature variations oraging. Comparator 655 compares the output of ramp generator 950 to theoutput of error amplifier 925. When the output of the ramp generator 950has reached the level of the error signal from error amplifier 925, thecomparator 655 trips and the output of comparator 945 goes high andresets flip-flop 670, thereby turning off the power switches 300, 250 inthe power converter stages, which remain off until the start of the nextswitching cycle.

In FIGS. 4-15, representative embodiments of the present disclosure areillustrated using analog or digital circuitry. It is to be understoodthat there are a plurality of implementation options for theillustrative embodiments of the present disclosure, all of which areconsidered equivalent and within the scope of the present disclosure. Inone embodiment of the present disclosure, controller 500 and theregulator are implemented using analog circuits such as amplifiers,comparators, integrators, error amplifiers, etc. In another embodimentof the present disclosure, controller 500 and the regulator areimplemented using digital circuits such as digital processors, memory,gates, FPGAs, etc. In another embodiment of the disclosure, controller500 and the regulator are implemented using a mixture of analog anddigital circuits.

As indicated above, the controller 500 (including variations 500A, 500B,and 500C, and any control logic block 650) may be any type of controlleror processor, and may be embodied as any type of digital logic, analogcircuitry, or other circuitry adapted to perform the functionalitydiscussed herein. As the term controller, processor, or control logicblock is used herein, a controller or processor or control logic blockmay include use of a single integrated circuit (“IC”), or may includeuse of a plurality of integrated circuits or other components connected,arranged, or grouped together, such as controllers, microprocessors,digital signal processors (“DSPs”), parallel processors, multiple coreprocessors, custom ICs, application specific integrated circuits(“ASICs”), field programmable gate arrays (“FPGAs”), adaptive computingICs, associated memory (such as RAM, DRAM and ROM), and other ICs andcomponents. As a consequence, as used herein, the term controller,processor, or control logic block should be understood to equivalentlymean and include a single IC, or arrangement of custom ICs, ASICs,processors, microprocessors, controllers, FPGAs, adaptive computing ICs,or some other grouping of integrated circuits or electronic componentswhich perform the functions discussed herein, with any associatedmemory, such as microprocessor memory or additional RAM, DRAM, SDRAM,SRAM, MRAM, ROM, PROM, FLASH, EPROM, or E²PROM. A controller orprocessor (such as controller 500, 500A, 500B, and 500C, or controllogic block 650), with its associated memory, may be adapted orconfigured (via programming, FPGA interconnection, or hard-wiring) toperform the methodology of the disclosure, as discussed above and below.For example, the methodology may be programmed and stored, in acontroller 500 or with its associated memory 630 and other equivalentcomponents, as a set of program instructions or other code (orequivalent configuration or other program) for subsequent execution whenthe controller or processor is operative (i.e., powered on andfunctioning). Equivalently, the controller or control logic block may beimplemented in whole or in part as FPGAs, digital logic such asregisters and gates, custom ICs and/or ASICs, the FPGAs, digital logicsuch as registers and gates, custom ICs or ASICs, also may be designed,configured and/or hard-wired to implement the methodology of thedisclosure. For example, the controller or processor may be implementedas an arrangement of controllers, microcontrollers, microprocessors,state machines, DSPs and/or ASICs, which are respectively programmed,designed, adapted, or configured to implement the methodology of thedisclosure, in conjunction with a memory 630.

The memory 630, which may include a data repository (or database), maybe embodied in any number of forms, including within any computer orother machine-readable data storage medium, memory device, or otherstorage or communication device for storage or communication ofinformation, currently known or which becomes available in the future,including, but not limited to, a memory integrated circuit (“IC”), ormemory portion of an integrated circuit (such as the resident memorywithin a controller or processor IC), whether volatile or non-volatile,whether removable or non-removable, including without limitation, RAM,FLASH, DRAM, SDRAM, SRAM, MRAM, FeRAM, ROM, EPROM or E²PROM, or anyother form of memory device, such as a magnetic hard drive, an opticaldrive, a magnetic disk or tape drive, a hard disk drive, othermachine-readable storage or memory media such as a floppy disk, a CDROM,a CD-RW, digital versatile disk (DVD) or other optical memory, or anyother type of memory, storage medium, or data storage apparatus orcircuit, which is known or which becomes known, depending upon theselected embodiment. In addition, such computer-readable media includesany form of communication media which embodies computer-readableinstructions, data structures, program modules, or other data in a datasignal or modulated signal. The memory 630 may be adapted to storevarious look-up tables, parameters, coefficients, other information anddata, programs, or instructions (of the software of the presentdisclosure), and other types of tables, such as database tables.

As indicated above, the controller or control logic block may beprogrammed, using software and data structures of the disclosure, forexample, to perform the methodology of the present disclosure. As aconsequence, the system and method of the present disclosure may beembodied as software which provides such programming or otherinstructions, such as a set of instructions and/or metadata embodiedwithin a computer-readable medium, discussed above. In addition,metadata may also be utilized to define the various data structures of alook-up table or a database. Such software may be in the form of sourceor object code, by way of example and without limitation. Source codefurther may be compiled into some form of instructions or object code(including assembly language instructions or configuration information).The software, source code, or metadata of the present disclosure may beembodied as any type of code, such as C, C++, C#, SystemC, LISA, XML,Java, ECMAScript, JScript, Brew, SQL and its variations (e.g., SQL 99 orproprietary versions of SQL), DB2, Oracle, or any other type ofprogramming language which performs the functionality discussed herein,including various hardware definition or hardware modeling languages(e.g., Verilog, VHDL, RTL) and resulting database files (e.g., GDSII).As a consequence, a “construct,” “program construct,” “softwareconstruct,” or “software,” as used equivalently herein, means and refersto any programming language, of any kind, with any syntax or signatures,which provides or can be interpreted to provide the associatedfunctionality or methodology specified (when instantiated or loaded intoa processor or computer and executed, including the controller 500, forexample).

The software, metadata, or other source code of the present disclosureand any resulting bit file (object code, database, or look-up table) maybe embodied within any tangible storage medium, such as any of thecomputer or other machine-readable data storage media, ascomputer-readable instructions, data structures, program modules, orother data, such as discussed above with respect to the memory 630,e.g., a floppy disk, a CDROM, a CD-RW, a DVD, a magnetic hard drive, anoptical drive, or any other type of data storage apparatus or medium, asmentioned above.

In some representative embodiments of the present disclosure, controlcircuitry is implemented using digital circuitry such as logic gates,memory registers, a digital processor such as a microprocessor ordigital signal processor, I/O devices, memory, analog-to-digitalconverters, digital-to-analog converters, FPGAs, etc. In otherrepresentative embodiments, this control circuitry is implemented inanalog circuitry such as amplifiers, resistors, integrators,multipliers, error amplifiers, operational amplifiers, etc. For example,one or more parameters stored in digital memory may, in an analogimplementation, be encoded as the value of a resistor or capacitor, thevoltage of a zener diode or resistive voltage divider, or otherwisedesigned into the circuit. It is to be understood that embodimentsillustrated as analog circuitry may alternatively be implemented withdigital circuitry or with a mixture of analog and digital circuitry andthat embodiments illustrated as digital circuitry may alternatively beimplemented with analog circuitry or with a mixture of analog anddigital circuitry within the scope of the present disclosure.

Memory 630 typically stores parameter values, controller 500 methods inthe form of software, data used by control logic block for computationsand executing software, etc. The memory 630 is utilized to store variousparameters and reference values, such as V_(LOW) _(—) _(RMS), ΔI_(L2),T_(MAX), I_(REF), D, I_(MAX), I_(MIN), initial and subsequentlydetermined values for the converter such as on-time (t_(ON)), off-time(t_(OFF)), converter switching period (T) duration (which may be interms of time or cycles), peak current values for the output current,the first power switch current, second stage current measurements,inductance values, various maximum voltage levels, etc. Variousparameters and reference values may be predetermined and pre-loaded inmemory 630. Examples of predetermined parameters that may be preloadedinto memory 630 include a maximum switching period T_(MAX), a maximumduty ratio D_(max), a minimum duty ratio Dmin, a desired output currentlevel I_(REF), a desired inductor current level, a minimum inputvoltage, a minimum RMS input voltage, a desired inductor ripple currentΔI_(L2), a desired output ripple current, etc. Other parameter andreference values may be received from the processor 630 and stored inmemory. The memory 630 may also provide various stored values directlyto the controller 500 or control logic 650, such as parameter valuest_(ON), T, etc.

In some embodiments, one or more of peripheral components, comprisingA/D converter 615, memory 630, any oscillator or clock (not separatelyillustrated), D/A converter 680, and I/O interface 675 are incorporatedas part of a controller 500 or regulator (600, 605, 900). Controller 500and/or control logic 650 execute methods of control as described in therepresentative embodiments of the present disclosure. The controller 500and/or control logic 650 may comprise any type of digital or sequentiallogic for executing the methodologies and performing selected operationsas discussed above and as further described below. For example, thecontroller 500 and/or control logic 650 may be implemented as one ormore finite state machines, various comparators, integrators,operational amplifiers, digital logic blocks, configurable logic blocks,or may be implemented to utilize an instruction set, and so on, asdescribed herein.

D/A converter 680 converts one or more control signals from controller500 and/or control logic 650 into an analog form and sends the one ormore control signals to the various gate drivers. Instead of or inaddition to D/A converter 680, ports on the I/O interface 675 may beused as controller 500 and/or control logic 650 outputs to the gatedrivers (290, 305, 306). Similarly, although sensor inputs may becoupled to A/D converter(s) 615 for digital implementations of acontroller 500, one or more of sensor inputs may be alternativelycoupled to inputs on the I/O interface 675 (inputs not shown). It is tobe noted that the term “I/O interface” is, for this purpose,interchangeable with terms “A/D converter” or “D/A converter,” and I/Ointerface 675, A/D converter 615, and D/A converter 680 all fall intothe class of I/O devices.

Numerous advantages of the representative embodiments of the presentdisclosure, for providing power to loads such as LEDs, are readilyapparent. The representative embodiments provide control over cascadedpower converters, while simultaneously reducing voltage stress. Therepresentative embodiments are capable of providing a plurality of typesof control over such power delivery, such as providing a substantiallyconstant current output, a hysteretic current output, and overshootprotection on start up. The representative embodiments utilize aplurality of sensors which may all be referenced to a common referencenode, such as ground, providing feedback signals and allowing forsimpler and more robust control electronics, which further enables moreaccurate and fine-tuned control over power delivery and circuitprotection, and enables an overall reduction in the size and cost of theconverter. Representative embodiments provide significant power factorcorrection, i.e., a power factor which is close to unity, when connectedto an AC line for input power, and further generate negligible harmonicsor electromagnetic interference.

Although the disclosure has been described with respect to specificembodiments thereof, these embodiments are merely illustrative and notrestrictive of the disclosure. In the description herein, numerousspecific details are provided, such as examples of electroniccomponents, electronic and structural connections, materials, andstructural variations, to provide a thorough understanding ofembodiments of the present disclosure. An embodiment of the disclosurecan be practiced without one or more of the specific details, or withother apparatus, systems, assemblies, components, materials, parts, etc.In other instances, structures, materials, or operations are notspecifically shown or described in detail to avoid obscuring aspects ofembodiments of the present disclosure. In addition, the various figuresare not drawn to scale and should not be regarded as limiting.

Reference throughout this specification to “one embodiment,” “anembodiment,” or a specific “embodiment” means that a particular feature,structure, or characteristic described in connection with the embodimentis included in at least one embodiment of the present disclosure and notnecessarily in all embodiments, and further, are not necessarilyreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics of any specific embodiment of the presentdisclosure may be combined in any suitable manner and in any suitablecombination with one or more other embodiments, including the use ofselected features without corresponding use of other features. Inaddition, many modifications may be made to adapt a particularapplication, situation, or material to the essential scope and spirit ofthe present disclosure. It is to be understood that other variations andmodifications of the embodiments of the present disclosure described andillustrated herein are possible in light of the teachings herein and areto be considered part of the spirit and scope of the claimed subjectmatter.

It will also be appreciated that one or more of the elements depicted inthe figures can also be implemented in a more separate or integratedmanner, or even removed or rendered inoperable in certain cases, as maybe useful in accordance with a particular application. Integrally formedcombinations of components are also within the scope of the disclosure,particularly for embodiments in which a separation or combination ofdiscrete components is unclear or indiscernible. In addition, use of theterm “coupled” herein, including in its various forms such as “coupling”or “couplable,” means and includes any direct or indirect electrical,structural, or magnetic coupling, connection or attachment, oradaptation or capability for such a direct or indirect electrical,structural or magnetic coupling, connection or attachment, includingintegrally formed components and components which are coupled via orthrough another component.

As used herein for purposes of the present disclosure, the term “LED”and its plural form “LEDs” should be understood to include anyelectroluminescent diode or other type of carrier injection- orjunction-based system which is capable of generating radiation inresponse to an electrical signal, including without limitation, varioussemiconductor- or carbon-based structures which emit light in responseto a current or voltage, light-emitting polymers, organic LEDs, and soon, including within the visible spectrum, or other spectra such asultraviolet or infrared, of any bandwidth, or of any color or colortemperature.

In the foregoing description and in the figures, sense resistors areshown in representative configurations and locations; however, othertypes and configurations of sensors may also be used and that sensorsmay be placed in other locations. Alternate sensor configurations andplacements are within the scope of the present disclosure.

As used herein, the term “DC” denotes both fluctuating DC (such as isobtained from rectified AC) and constant voltage DC (such as is obtainedfrom a battery, voltage regulator, or power filtered with a capacitor).As used herein, the term “AC” denotes any form of alternating currentwith any waveform (sinusoidal, sine squared, rectified sinusoidal,square, rectangular, triangular, sawtooth, irregular, etc.) and with anyDC offset and may include any variation such as chopped or forward- orreverse-phase modulated alternating current, such as from a dimmerswitch.

With respect to sensors, we refer herein to parameters that “represent”a given metric or are “representative” of a given metric, where a metricis a measure of a state of at least part of the regulator or its inputsor outputs. For example, we note that I_(LOAD) “represents” the loadcurrent and we say that inductor current I_(L2) may be used to“represent” the load current. A parameter is considered to represent ametric if it is related to the metric directly enough that regulatingthe parameter will satisfactorily regulate the metric. For example, themetric of LED current may be represented by I₂, the average current ofinductor L₂, because they are similar and because regulating I_(L2)satisfactorily regulates I_(LED). In the case of output current (such asLED current), a parameter is considered to represent output current ifit is related to the output current directly enough that regulating theparameter will satisfactorily regulate the output current. A parametermay be considered to be an acceptable representation of a metric if itrepresents a multiple or fraction of the metric. It is to be noted thata parameter may physically be a voltage and yet still represents acurrent value. For example, the voltage across a sense resistor“represents” current through the resistor.

In the foregoing description of illustrative embodiments and in attachedfigures where diodes are shown, it is to be understood that synchronousdiodes or synchronous rectifiers (for example, relays or MOSFETs orother transistors switched off and on by a control signal) or othertypes of diodes may be used in place of standard diodes within the scopeof the present disclosure. Representative embodiments presented heregenerally generate a positive output voltage with respect to ground;however, the teachings of the present disclosure apply also to powerconverters that generate a negative output voltage, where complementarytopologies may be constructed by reversing the polarity ofsemiconductors and other polarized components.

For convenience in notation and description, transformers such as firsttransformer 230 are referred to as a “transformer,” although inillustrative embodiments, it behaves in many respects also as aninductor. In fact, in alternative embodiments, first transformer 230 isreplaced with one or more simple inductors by making appropriateadjustments to the circuit topology, with FIG. 7 as an example.Similarly, inductors can, under proper conditions, be replaced bytransformers. We refer to transformers and inductors as “inductiveelements,” with the understanding that they perform similar functionsand may be interchanged within the scope of the present disclosure.

Furthermore, any signal arrows in the drawings/figures should beconsidered only representative, and not limiting, unless otherwisespecifically noted. Combinations of components of steps will also beconsidered within the scope of the present disclosure, particularlywhere the ability to separate or combine is unclear or foreseeable. Thedisjunctive term “or,” as used herein and throughout the claims thatfollow, is generally intended to mean “and/or,” having both conjunctiveand disjunctive meanings (and is not confined to an “exclusive or”meaning), unless otherwise indicated. As used in the description hereinand throughout the claims that follow, “a,” “an,” and “the” includeplural references unless the context clearly dictates otherwise. Also asused in the description herein and throughout the claims that follow,the meaning of “in” includes “in” and “on” unless the context clearlydictates otherwise.

The foregoing description of illustrated embodiments of the presentdisclosure, including what is described in the summary or in theabstract, is not intended to be exhaustive or to limit the disclosure tothe precise forms disclosed herein. From the foregoing, it will beobserved that numerous variations, modifications, and substitutions areintended and may be effected without departing from the spirit and scopeof the claimed subject matter. It is to be understood that no limitationwith respect to the specific methods and apparatus illustrated herein isintended or should be inferred. It is, of course, intended to cover bythe appended claims all such modifications as fall within the scope ofthe claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. An apparatus for powerconversion, the apparatus comprising: a first power converter stageincluding a first power switch; a second power converter stage coupledto the first power converter stage, wherein the second power converterstage includes a second power switch, and wherein the second powerconverter stage is couplable to provide an output current to a load; afirst sensor configured to sense an electrical parameter; and acontroller coupled to the first power switch, the second power switch,and the first sensor, wherein the controller is configured to turn thefirst and second power switches into an on-state at a frequencysubstantially corresponding to a switching period.
 2. The apparatus ofclaim 1, wherein the controller is further configured to determine theswitching period as a switching interval that maintains voltage stressof the first power switch and the second power switch belowcorresponding predetermined levels.
 3. The apparatus of claim 1, whereinthe controller is further configured to maintain a switching duty cyclewithin a predetermined range to maintain voltage stress of the firstpower switch and the second power switch below correspondingpredetermined levels.
 4. The apparatus of claim 1, wherein thecontroller is further configured to turn the first and second powerswitches into the on-state and into an off-state substantiallyconcurrently.
 5. The apparatus of claim 1, wherein the first sensor iscoupled to the second power converter stage, and wherein the electricalparameter is a current level corresponding to the output current.
 6. Theapparatus of claim 5, wherein the first power converter stage iscouplable to receive an input voltage, and wherein the apparatus furthercomprises: a second sensor coupled to the first power converter stage,wherein the second sensor is configured to sense an input voltage level.7. The apparatus of claim 6, wherein the controller is furtherconfigured to turn the first and second power switches into the on-stateand into an off-state with the switching period determined in responseto the electrical parameter.
 8. The apparatus of claim 6, wherein thecontroller is further configured to turn the first and second powerswitches into an off-state in response to an increase in the electricalparameter to a first threshold.
 9. The apparatus of claim 8, wherein thecontroller is further configured to determine the first threshold basedon a predetermined reference current level, a predetermined currentvariance level, a minimum input voltage level, and the sensed inputvoltage level.
 10. The apparatus of claim 8, wherein the controller isfurther configured to determine the first threshold (I_(MAX)) assubstantially proportional to a predetermined reference current level(I_(REF)) plus an offset term comprising a predetermined currentvariance level (ΔI_(L2)) multiplied by the square of a minimum RMS inputvoltage (V² _(LOW) _(—) _(RMS)) and divided by twice the square of anRMS value of the sensed input voltage (2*V² _(IN) _(—) _(RMS)), namely(I_(MAX)∝I_(REF)+(ΔI_(L2)*V² _(LOW) _(—) _(RMS)/2*V² _(IN) _(—)_(RMS))).
 11. The apparatus of claim 8, wherein the controller isfurther configured to turn the first and second power switches into theon-state in response to a decrease in the electrical parameter to asecond threshold.
 12. The apparatus of claim 11, wherein the controlleris further configured to determine the second threshold based on apredetermined reference current level, a predetermined current variancelevel, a minimum input voltage level, and the sensed input voltagelevel.
 13. The apparatus of claim 11, wherein the controller is furtherconfigured to determine the second threshold (I_(MIN)) as substantiallyproportional to a predetermined reference current level (I_(REF)) minusan offset term comprising a predetermined current variance level(ΔI_(L2)) multiplied by the square of a minimum RMS input voltage (V²_(LOW) _(—) _(RMS)) and divided by twice the square of an RMS value ofthe sensed input voltage (2*V² _(IN) _(—) _(RMS)), namely(I_(MIN)∝I_(REF)−(ΔI_(L2)*V² _(LOW) _(—) _(RMS)/2*V² _(IN) _(—)_(RMS))).
 14. The apparatus of claim 11, further comprising: a memoryconfigured to store a look-up table, wherein the look-up table includesa plurality of first and second thresholds corresponding to a pluralityof input voltage levels.
 15. The apparatus of claim 14, wherein thecontroller is further configured to determine the first threshold andthe second threshold by accessing the look-up table using the sensedinput voltage level.
 16. The apparatus of claim 6, wherein the switchingperiod corresponds to a first threshold and to a second threshold, andwherein the controller is further configured to turn the first andsecond power switches into an off-state in response to an increase inthe electrical parameter to the first threshold and into the on-state inresponse to a decrease in the electrical parameter to the secondthreshold.
 17. The apparatus of claim 6, wherein the controller isfurther configured to determine the switching period in response to thesensed input voltage level.
 18. The apparatus of claim 17, furthercomprising: a memory configured to store a look-up table, wherein thelook-up table includes a plurality of switching period valuescorresponding to a plurality of input voltage levels.
 19. The apparatusof claim 18, wherein the controller is further configured to determinethe switching period by accessing the look-up table using the sensedinput voltage level.
 20. The apparatus of claim 6, wherein thecontroller is further configured to determine the switching period basedon a maximum switching period, a minimum input voltage, and the sensedinput voltage level.
 21. The apparatus of claim 6, wherein thecontroller is further configured to determine the switching period (T)as substantially proportional to a maximum switching period (T_(MAX))multiplied by the square of a minimum RMS input voltage and divided bythe square of an RMS value of the sensed input voltage level, namely,(T∝T_(MAX)*V² _(LOW) _(—) _(RMS)/V² _(IN) _(—) _(RMS)).
 22. Theapparatus of claim 6, wherein the controller is further configured todetermine the switching period (T) as substantially proportional to aminimum switching period (T_(MIN)) multiplied by the square of a maximumRMS input voltage and divided by the square of an RMS value of thesensed input voltage level, namely, (T∝T_(MIN)*V² _(HIGH) _(—) _(RMS)/V²_(IN) _(—) _(RMS)).
 23. The apparatus of claim 6, wherein the controlleris further configured to determine the switching period (T) assubstantially proportional to a selected switching period (T₁)multiplied by the square of a selected RMS input voltage (V² ₁ _(—)_(RMS)) and divided by the square of an RMS value of the sensed inputvoltage level, namely, (T∝T₁*V² ₁ _(—) _(RMS)/V² _(IN) _(—) _(RMS)). 24.The apparatus of claim 5, wherein the controller is further configuredto decrease the duty cycle in response to the output current being abovea first predetermined level.
 25. The apparatus of claim 5, wherein thecontroller is further configured to increase the duty cycle in responseto the output current being below a second predetermined level.
 26. Theapparatus of claim 1, further comprising: a memory configured to store apredetermined parameter, wherein the predetermined parameter includesone of the following parameters: a minimum switching period, a maximumswitching period, a maximum duty ratio, a minimum duty ratio, a desiredoutput current level, a desired inductor current level, a maximum inputvoltage, a minimum input voltage, a minimum RMS input voltage, a desiredinductor ripple current, a desired output ripple current, or a maximumvoltage stress level.
 27. The apparatus of claim 1, wherein the firstpower converter stage further comprises a flyback configuration, a buckconfiguration, or a double-buck configuration.
 28. The apparatus ofclaim 1, wherein the second power converter stage further comprises abuck configuration, a boost configuration, or a buck-boostconfiguration.
 29. The apparatus of claim 1, wherein the first powerconverter stage further comprises a transformer and the second powerconverter stage further comprises an inductor.
 30. The apparatus ofclaim 1, wherein the first power converter stage further comprises afirst inductor and the second power converter stage further comprises asecond inductor.
 31. The apparatus of claim 6, wherein the first sensorand the second sensor are referenced to a common reference.
 32. Theapparatus of claim 31, wherein the common reference is a groundpotential.
 33. The apparatus of claim 1, wherein the load comprises alight-emitting diode.
 34. The apparatus of claim 1, wherein thecontroller is further configured to operate the first power converterstage in a discontinuous conduction mode and to operate the second powerconverter stage in a continuous conduction mode.
 35. The apparatus ofclaim 1, wherein the first power converter stage is couplable to receivean input voltage having a predetermined range of voltages.
 36. Theapparatus of claim 35, wherein the predetermined range of voltages issubstantially from 90 V RMS to 264 V RMS.
 37. The apparatus of claim 35,wherein the first power converter stage further comprises a rectifiercouplable to receive an AC input voltage.
 38. The apparatus of claim 1,wherein the first power converter stage further comprises a diode, andwherein the controller is further configured to determine a switchingperiod as a switching interval that maintains voltage stress of thediode below a predetermined level or is configured to maintain aswitching duty cycle within a predetermined range to maintain voltagestress of the diode below the predetermined level.
 39. The apparatus ofclaim 1, wherein the first power converter stage is couplable to receivean input voltage, wherein the first sensor is coupled to the first powerconverter stage, and wherein the electrical parameter is an inputvoltage level corresponding to the input voltage.
 40. A method ofproviding power conversion for a load using a power converter, themethod comprising: sensing a first parameter including an input voltagelevel of an input voltage to the power converter, wherein the powerconverter comprises a first power converter stage coupled to a secondpower converter stage, wherein the first power converter stage includesa first power switch, and wherein the second power converter stageincludes a second power switch; sensing a second parameter, wherein thesecond parameter includes an output current of the power converter or acurrent of the second power switch; turning the first and second powerswitches into an on-state substantially concurrently and at a frequencysubstantially corresponding to a switching period; and turning the firstand second power switches into an off-state substantially concurrentlywhile maintaining a switching duty cycle within a predetermined range.41. The method of claim 40, wherein the switching duty cycle issubstantially constant.
 42. The method of claim 40, wherein theswitching period corresponds to a first threshold and to a secondthreshold, wherein said turning the first and second power switches intoan off-state comprises turning the first and second power switches intothe off-state in response to an increase in the first parameter to thefirst threshold, and wherein said turning the first and second powerswitches into an on-state comprises turning the first and second powerswitches into the on-state in response to a decrease in the firstparameter to the second threshold.
 43. The method of claim 42, furthercomprising: using a predetermined reference current level, apredetermined current variance level, a minimum input voltage level, andthe sensed input voltage level, determining the first threshold and thesecond threshold.
 44. The method of claim 42, further comprising: usingthe sensed input voltage level, accessing a memory to determine thefirst threshold and the second threshold.
 45. The method of claim 40,further comprising: using the sensed input voltage level, accessing amemory to determine the switching period.
 46. The method of claim 40,further comprising: using a maximum switching period, a minimum inputvoltage level, and the sensed input voltage level, determining theswitching period.
 47. The method of claim 40, further comprising:decreasing the switching duty cycle in response to the output currentbeing above a first predetermined level; and increasing the switchingduty cycle in response to the output current being below a secondpredetermined level.
 48. The method of claim 40, further comprising:operating the first power converter stage in a discontinuous conductionmode; and operating the second power converter stage in a continuousconduction mode.
 49. The method of claim 40, further comprising:receiving the input voltage having a predetermined range of voltagelevels.
 50. A computer-readable medium having instructions storedthereon that, in response to execution by a computing device, cause thecomputing device to: sense a first parameter comprising an input voltagelevel of an input voltage to a power converter, wherein the powerconverter comprises a first power converter stage coupled to a secondpower converter stage, wherein the first power converter stage includesa first power switch, and wherein the second power converter stageincludes a second power switch; sense a second parameter comprising anoutput current of the power converter or a current of the second powerswitch; turn the first power switch and the second power switch into anon-state substantially concurrently and at a frequency substantiallycorresponding to a switching period; and turn the first and second powerswitches into an off-state substantially concurrently while maintaininga switching duty cycle within a predetermined range.
 51. Thecomputer-readable medium of claim 50, further comprising instructionsthat, in response to execution by the computing device, cause thecomputing device to: turn the first and second power switches into theoff-state in response to an increase in the first parameter to a firstthreshold, wherein the switching period corresponds to the firstthreshold and a second threshold; and turn the first and second powerswitches into the on-state in response to a decrease in the firstparameter to the second threshold.
 52. The computer-readable medium ofclaim 51, further comprising instructions that, in response to executionby the computing device, cause the computing device to determine thefirst threshold and the second threshold using a predetermined referencecurrent level, a predetermined current variance level, a minimum inputvoltage level, and the sensed input voltage level.
 53. Thecomputer-readable medium of claim 51, further comprising instructionsthat, in response to execution by the computing device, cause thecomputing device to access a memory to determine the first threshold andthe second threshold using the sensed input voltage level.
 54. Thecomputer-readable medium of claim 50, further comprising instructionsthat, in response to execution by the computing device, cause thecomputing device to access a memory to determine the switching periodusing the sensed input voltage level.
 55. The computer-readable mediumof claim 50, further comprising instructions that, in response toexecution by the computing device, cause the computing device todetermine the switching period using a maximum switching period, aminimum input voltage level, and the sensed input voltage level.
 56. Thecomputer-readable medium of claim 50, further comprising instructionsthat, in response to execution by the computing device, cause thecomputing device to: decrease the switching duty cycle in response tothe output current being above a first predetermined level; and increasethe switching duty cycle in response to the output current being below asecond predetermined level.
 57. The computer-readable medium of claim50, further comprising instructions that, in response to execution bythe computing device, cause the computing device to: operate the firstpower converter stage in a discontinuous conduction mode; and operatethe second power converter stage in a continuous conduction mode.
 58. Asystem for power conversion, the system comprising: a plurality ofloads; a first power converter stage configured to have a flybackconfiguration, wherein the first power converter stage includes a firstpower switch; a first sensor coupled to the first power converter stage,wherein the first sensor is configured to sense an input voltage levelof the first power converter stage; a second power converter stagehaving a buck configuration and coupled to the first power converterstage, wherein the second power converter stage includes a second powerswitch and an inductor, and wherein the second power converter stage iscoupled to the plurality of loads to provide an output current to theplurality of loads; a second sensor coupled to the second powerconverter stage, wherein the second sensor is configured to sense anoutput current level or a current level in the inductor; and acontroller coupled to the first power switch, the second power switch,the first sensor, and the second sensor, wherein the controller isconfigured to turn the first and second power switches into an on-stateat a frequency substantially corresponding to a switching period whilemaintaining a switching duty cycle within a predetermined range.